The municipal transit worker who the police say shot and killed nine of his colleagues at a rail yard in San Jose was stopped by border officials in 2016, during which they searched his bags and found writings about how he hated the agency he worked for, according to an official who described a message that was circulated within the Homeland Security Department after Wednesday’s shooting.

The gunman, who the authorities said killed himself and was identified as Samuel James Cassidy, 57, was stopped by Customs and Border Protection officials in August 2016 as he returned from a trip to the Philippines, the official said, speaking on the condition of anonymity.

In addition to the notebook with writings about how he hated the Santa Clara Valley Transportation Authority, where he worked, the officials also found books about terrorism and manifestoes, the official said.

The police in California said on Thursday that they were still trying to understand what led to the shooting, but that they had discovered more firepower at the scene than they had initially reported.

Three semiautomatic handguns were found at the scene, as were 32 high-capacity magazines which each held a dozen nine-millimeter rounds, said Deputy Cian Jackson, a spokesman for the Santa Clara County Sheriff’s Office. He said at least one witness had reported that the gunman told someone at the scene that he was not going to hurt them, suggesting that he may have selected his victims.

Detectives were continuing to interview witnesses on Thursday to try to understand what had happened, though a statement from the sheriff’s office said the gunman had been “a highly disgruntled V.T.A. employee for many years.”

“The general vibe we get is that he didn’t like anybody and nobody really liked him,” Deputy Jackson said in an interview. He added that the police had feared that there was an explosive device in the gunman’s locker at work, but that none was discovered.

He declined to comment on the report of the 2016 stop by border officials, which was earlier reported by The Wall Street Journal. The Homeland Security Department, which includes Customs and Border Protection, also declined to comment on the stop and did not respond to a question about whether the border agency had shared information about it at the time.

The gunman had carried out the rampage early on Wednesday morning at the same time that his house was erupting in flames more than eight miles away.

Sheriff Laurie Smith told reporters that based on photographs of at least two of the guns, they appeared to be legal to buy in California, though she did not say whether Mr. Cassidy had purchased them legally. Californians voted in 2016 to ban magazines that hold more than 10 rounds, as the gunman’s did, but it was blocked from going into effect by a judge after a lawsuit by the National Rifle Association. A legal battle over the law is ongoing.

Sheriff Smith said the gunman had been “very deliberate” in killing his colleagues at the transit agency near downtown San Jose, and that he had killed himself as deputies rushed into the rail yard from their headquarters next door.

Sheriff Smith said on NBC’s “Today” show that the fire at Mr. Cassidy’s house was first reported three minutes after someone had called 911 to report gunfire at the rail yard, suggesting that he may have set off “some kind of device” to start the blaze as he opened fire. She said investigators had found “explosive materials” at the suspect’s home.

The nine victims of the shooting were identified by the Santa Clara County medical examiner’s office as Paul Delacruz Megia, 42; Taptejdeep Singh, 36; Adrian Balleza, 29; Jose Dejesus Hernandez III, 35; Timothy Michael Romo, 49; Michael Joseph Rudometkin, 40; Alex Ward Fritch, 49; Abdolvahab Alaghmandan, 63; and Lars Kepler Lane, 63.

The phone call came at 6:36 a.m. and lasted for 44 seconds. Sukhvir Singh, a mechanic at the rail yard where nine of his colleagues were killed, says the call on Wednesday morning was the difference between life and death for him and a half dozen co-workers.

On the line was a colleague, Taptejdeep Singh, with an urgent warning.

“He was talking quite fast,” Sukhvir Singh said. “He said, ‘Hey! There’s an active shooter. Get out.’”

Sukhvir Singh, who specializes in repairing and maintaining the light-rail trains that run through San Jose, fled with his crew members to a windowless building that houses antique rolling stock. There they waited until sheriff’s deputies arrived.

The man who made the call was killed by the gunman.

Sukhvir Singh said he was told that his body was discovered on a staircase.

Few employees at the Santa Clara Valley Transportation Authority, the agency where the shooting took place, have told their stories of survival. Outside a union hall on Thursday, rank-and-file members said they had been told they would be fired if they spoke to reporters.

Sukhvir Singh said he wanted to speak out as a tribute to Taptejdeep Singh, whom he described as an unfailingly gracious and helpful colleague. The two men are not related.

“There are still people out there who want to help others more than themselves,” he said. “He is the hero for everyone.”

The killing of Taptejdeep Singh, he said, also underlined the senselessness of the shooting.

He said the gunman, identified as Samuel Cassidy, barely knew Taptejdeep Singh. They worked in different departments. Mr. Cassidy worked in Building B, which handles the maintenance of electrical substations. Taptejdeep Singh was a light-rail operator who when he was not driving trains was in Building A, where trains where serviced.

“They didn’t have any connection at all,” he said.

For a time Sukhvir Singh worked in the same building as Mr. Cassidy. He would pass him in the halls and say hello. At best Mr. Cassidy would acknowledge him with a grunt, he said.

“He didn’t really communicate with other people,” he said. “He was in his own world.”

Nine employees of the Santa Clara Valley Transportation Authority were killed in a shooting at the system’s rail yard in San Jose on Wednesday. Here is what we know about some of their lives.

Adrian Balleza, 29

Adrian Balleza was described by his wife, Heather Balleza, as a humble and caring individual who was loved by many people. Now, she wrote over a messaging platform on Thursday morning, his family and friends are heartbroken. Mr. Balleza’s 2-year-old son will have to spend his years without a father. “It still doesn’t feel real,” she wrote.

Mr. Balleza started working at the V.T.A. in 2014 as a bus operator trainee, later becoming a maintenance worker and light rail operator, the authority said.

Mr. Balleza could not wait until his son was old enough for them to go fishing together, Ms. Balleza said. She was grief-stricken that her husband would not be able to watch his son grow up. And her own world is no longer whole, she said.

“The world needs more people like my husband, not one less,” she said. “He was my night and day. The best father and husband … my forever angel.”

Naunihal Singh, the superintendent of light rail transportation for V.T.A., was Mr. Balleza’s supervisor. He described Mr. Balleza as a “gem of a person” and a “very kindhearted” man who was always volunteering to help organize fun activities for co-workers.

“Words are not enough to justify the pain we’re all going through,” Mr. Singh told reporters on Thursday. “I’m angry. I’m sad. I’m at a loss for words.”

Lars Kepler Lane, 63

Ed Lane expressed anguish on Wednesday night over the death of his brother Lars Lane, who worked as a journeyman lineman for the V.T.A., according to his LinkedIn profile.

“My brother was murdered today,” Mr. Lane said in an email. “Not by a gun but by a man that could have been helped.”

Mr. Lane spent much of the day waiting to find out if his brother, who local media outlets reported was a husband and a father, was among the victims. He sharply criticized the way the notification process was handled, in addition to the renewed call for tougher gun control laws in the aftermath of another mass shooting.

“I’m tired of the gun control propaganda,” he said. “Politicians and law enforcement patting themselves on the back leaving my family in the dark for 12 hours. The family assistance was absolutely a front of incompetence.”

Paul Delacruz Megia, 42

Paul Delacruz Megia immigrated to the United States from the Philippines when he was a toddler, according to his father, Leonard Megia. He had two sons, a daughter, and a stepson, and he loved them deeply, his father said.

They liked to take a boat out and go wakeboarding during the summer, his father said, and in the winter, snowboarding was their favorite activity.

Mr. Megia had planned a trip to Disneyland with his children. They were scheduled to leave on Thursday.

“He was a wonderful dad,” his father said. “He’s my son and my best friend.”

Mr. Megia and his father lived in the same home together near Tracy, Calif., along with his three children. The father and son were very close — they enjoyed fishing and spending time in the snow together during the winter. Mr. Megia was always smiling, his father said, and constantly had a positive demeanor.

The V.T.A. said that Mr. Megia had been employed there for 20 years, working his way up from bus operator trainee to superintendent-service management. Mr. Megia left home every morning at 4:30 a.m. to get to work on time, but made sure to call his children every single morning to check in on them before they started school.

“He’s a very loving dad who cared a lot about his children,” his father said. “They’re going to miss him.”

Mr. Singh, the superintendent of light rail transportation for the V.T.A., shared an office with Mr. Megia. He described Mr. Megia as an easygoing manager who was popular with employees.

“Sometimes my demands could be unreasonable, but Paul always accepted it with a smile. He always was willing to help his employees,” Mr. Singh said. “They seemed to reach out to him for whatever their needs were.”

Taptejdeep Singh, 36

Taptejdeep Singh, a light rail operator for the V.T.A., was remembered by a cousin as the nicest person in his family and a gregarious man who enjoyed playing volleyball.

“We are very sad right now,” said the cousin, Bagga Singh, who was one of more than a dozen family members waiting all day to learn what had happened to their relative. Shortly after 6 p.m., they got the bad news. Several family members broke down sobbing at a Red Cross facility and were escorted away.

The death of Mr. Singh, who was Sikh and moved to the United States from India in 2005, marks the second time in two months that members of the country’s Sikh community mourned after a mass shooting. In April, four Sikhs were among the eight people killed in a shooting at a FedEx warehouse in Indianapolis.

Taptejdeep Singh had a wife and two young children, Bagga Singh said, and enjoyed his job at the V.T.A., where he had been working for eight or nine years. He also had an insurance license and was a real estate agent, Bagga Singh said.

“He can work anything he wants, very smart guy,” he said.

Family members said county officials told them that Taptejdeep Singh acted heroically when he detected danger during the attack, calling out to his co-workers that shots were being fired and quickly ushering one woman into a secure room.

“I think he’s the one who tried to save the people, as many as he could,” said Bagga Singh, who also spoke out against gun violence: “Nobody should have a gun.”

Michael Joseph Rudometkin, 40

Michael Joseph Rudometkin began working for the V.T.A. in 2013 as a transit mechanic, according to the agency. He then became an electro-mechanic and an overhead line worker.

Raul Peralez, a member of the San Jose City Council, called Mr. Rudometkin a “lifelong friend.” He said that he and his father had been planning a golf outing with Mr. Rudometkin.

“Now that will never happen again,” Mr. Peralez said on Facebook. “My family and I have lost a long time great friend and there are no words to describe the heartache we are feeling right now, especially for his family.”Mr. Rudometkin was married and is survived by his parents and his sister, said Mr. Peralez, who told reporters on Thursday that he had met with Mr. Rudometkin’s wife on Wednesday night. Mr. Peralez said he visited with Mr. Rudometkin’s parents, sister and brother-in-law Thursday morning.

“I truly feel for all the victims’s families,” he said. “Personally, my heart is broken.”

Abdolvahab Alaghmandan, 63

Mr. Alaghmandan, an immigrant from Iran, was a substation maintainer who worked at the transit agency for about two decades.

A family friend called him “like a second father of my own.”

“He moved his family to the United States so that they could have a better life, which makes this horrific tragedy just all the worse,” said Megan Staker, whose boyfriend, Soheil, is Mr. Alaghmandan’s son.

“He worked so hard so that his family could have a good life. He was so funny, and kind and loving, and could fix anything. Things will never be the same without him. He brought so much joy and laughter to our lives. To say he will be missed is an understatement. Our hearts are forever broken.”

This briefing will continue to be updated as we learn more about the victims.

A leader in a labor group that represents the workers killed at the rail yard in San Jose this week expressed frustration and bewilderment about the gunman’s motives on Thursday.

“No issues that we know of that were red flags — but what is a red flag?” said Arturo Aguilar, chairman of the California Conference Board of the Amalgamated Transit Union, in an interview on the lawn outside the union’s San Jose office. “We are not trained to analyze co-workers.”

Mr. Aguilar said the union did not see patterns in which of his colleagues that Samuel James Cassidy, 57, a maintenance worker for the Santa Clara Valley Transportation Authority, chose to shoot. The nine victims who were killed on Wednesday came from various departments, he said.

“We don’t know the relationships or the correlations between the victims and the shooter,” Mr. Aguilar said. “And we will never know.”

The union’s main task right now, Mr. Aguilar said, was consoling members, many of whom gathered at the union’s office on Thursday.

Investigators do not yet know whether the victims of Wednesday’s shooting at a rail yard in San Jose were specifically targeted, and they have not identified a motive for the gunman, Laurie Smith, the sheriff of Santa Clara County, said in an interview Thursday morning on NBC’s “Today.”

“What in the world could possibly prompt someone to take this kind of action?” she said. “We don’t know at this point.”

Officers confronted Samuel James Cassidy, whom California law enforcement officials identified as the gunman, “within a few minutes,” Sheriff Smith said, suggesting the carnage could have been worse if they had not arrived quickly. The sheriff’s office headquarters is next door to the rail yard.

Mr. Cassidy carried two semiautomatic handguns and had 11 loaded magazines, she said, adding that police dogs at the scene found materials for bombs in what investigators believe to be his locker.

Investigators are also looking into how a fire at Mr. Cassidy’s house was apparently started while he was carrying out the attack. The police received the first report of shots fired at 6:34 a.m., and the call reporting the fire came in at 6:37 a.m.

“What we’re operating under now, but I’m not sure that this isn’t going to change, is that he set some kind of a device to go off at a certain time, probably to coincide with the shooting,” she said.

The nine victims of the shooting were spread over two buildings, she said.

“He was very deliberate, very fast,” she said. “He knew where employees would be.”

Everything was set for an ordinary day in a suburban corner of southeastern San Jose on Wednesday. Doug Suh had an early golf game scheduled. Andy and Alice Abad were preparing for a morning doctor’s appointment. And Anthony Nguyen was, as always, set for his daily 9:30 a.m. church service.

But by early afternoon, each resident of the Evergreen neighborhood discovered in their own way that something had gone very wrong.

Early on Wednesday, Abad watched from his kitchen as a funnel of smoke and flames poured from a home one block away. Suh got a call from a friend on the golf course about a man who had killed eight workers at the Santa Clara Valley Transportation Authority rail yard. (It would later become nine.) And Nguyen got a similar call from a friend but with a more disturbing detail: “The man who did it was your neighbor,” Nguyen’s friend told him.

By noon, their neighborhood was swarming with multiple fire and police vehicles, federal agents and a boxy blue truck from the San Jose bomb squad. Men with gas masks and oxygen tanks stood amid the flashing lights in the cul-de-sacs of what they all described as a quiet suburban neighborhood that is home largely to Vietnamese and Filipino immigrants.

Investigators on Thursday morning were still piecing together the havoc unleashed the day before.

The suspect in the shooting, Samuel James Cassidy, 57, a maintenance worker who had been with the V.T.A. for at least a decade, lived in a one-story home with white trim and a patchy lawn in the Evergreen neighborhood.

Officials said Wednesday that they believed Cassidy was responsible for both setting his home on fire and then proceeding to shoot his colleagues at his workplace, the railway yard eight miles from where he lived.

Suh, who lives across the street from the suspect, scanned through his security camera footage when he returned home from his golf game. The camera captured Cassidy at 5:40 a.m. loading his white pickup truck with a black bag. He was wearing a uniform with reflective stripes.

Nguyen, a retired real estate broker who has lived in San Jose for the past four decades, said he was baffled by what had happened.

“Everything has been very perfect,” he said of his neighborhood. “People are nice and quiet here.”

When he saw flashing lights on the corner of his street on Wednesday, he assumed there had been a traffic accident. Then his friend called with the news about the shooting.

“What about all these families that lost sons and fathers?” Mr. Nguyen asked in his driveway. “I’m so sorry for them. It’s not right. All these broken hearts.”

California has some of the most progressive gun laws in the country and is one of two states to receive a full A rating from the Giffords Law Center, which advocates for gun reform. The state requires universal background checks for gun owners and restrictions on the size of magazines, along with other laws that restrict the types of firearms that a person can legally purchase.

But after a mass shooting on Wednesday that claimed nine lives at a San Jose rail yard, politicians including Gov. Gavin Newsom and President Biden urged lawmakers to take further action in legislating firearms. In a statement, Mr. Biden urged Congress to “heed the call of the American people, including the vast majority of gun owners, to help end this epidemic of gun violence.”

It is unclear what type of weapons were used by the gunman, whose body was found at the scene, where he had acquired them, or whether they would have met California’s legal standards. The state bans the possession of assault weapons, with some exceptions, and bans so-called ghost guns, which are typically assembled by an individual and do not contain a serial number.

According to the Giffords Law Center, which also gives an A rating to New Jersey, California has the seventh-lowest rate of gun deaths in the country, and has the most robust system for taking guns from people who are barred from having them.

At least one of California’s gun laws is being challenged in court. In August 2020, a divided three-judge panel of the U.S. Court of Appeals for the Ninth Circuit struck down a state law that banned the possession of magazines that can hold more than 10 rounds. In February, however, the court said it would reconsider that decision.

The man identified by California law enforcement officials as the gunman in a mass shooting at a San Jose rail yard on Wednesday lived alone and had a hostile personality, according to interviews with a neighbor and an ex-girlfriend, as well as a review of court records.

The gunman, Samuel James Cassidy, 57, lived southeast of downtown San Jose, in a suburban neighborhood of cul-de-sacs and palm trees, public records show. Doug Suh, a real estate agent who lived across the street, described Mr. Cassidy, who authorities say appeared to have killed himself, as someone with a short temper.

“I was afraid of him,” Mr. Suh said. “My wife was scared of him, too.”

“He lived alone,” Mr. Suh said. “I never saw any friends or family. I never saw anyone else going into the house.”

Mr. Suh recalled Mr. Cassidy once lashing out at him when Mr. Suh turned his car around in Mr. Cassidy’s driveway. “He yelled, ‘Do not come onto my driveway.’”

Family court records show that Mr. Cassidy was married for 10 years before divorcing in 2004. The couple had no children.

In 2009, Mr. Cassidy sought a restraining order against his former girlfriend, Jingkun Wang, known as Connie. In court filings, he accused her of vandalizing his roommate’s car, calling at late hours of the night, hurling insults and suggesting she had him under surveillance.

Ms. Wang countered that Mr. Cassidy, whom she had dated for a year, had “major mood swings due to bipolar disorder” which were exacerbated when he consumed large quantities of alcohol. She also accused him of forcing himself on her sexually.

Their relationship failed when he told her around February 2009 that a female houseguest had moved in, and that she should no longer come visit.

She denied vandalizing any vehicles, and accused him of stealing items from restaurants and employers, and failing to return her television and computer printer.

“He has manipulated me in many ways which are apparent to my friends and family,” she said.

The court ordered Ms. Wang to stay at least 300 yards from him, his parents and his new girlfriend for three years.

In a phone interview on Wednesday, Ms. Wang, now 58, said she had not spoken to Mr. Cassidy in 12 years, but described him as someone who was “not mentally stable,” who would be loving one moment and mean the next. She also described his drinking as excessive.

They met on Match.com, Ms. Wang said. Two months into their relationship, he told her to pick a diamond ring and he would buy it, she recalled. She declined.

After they broke up, Mr. Cassidy showed up at her apartment complex one day, Ms. Wang said. A neighbor let him in and he took her brand-new Toyota Camry, for which he had a key, without permission. He returned it damaged from what he told her was an accident, she said.

The Gun Violence Archive, which defines a mass shooting as one with four or more people injured or killed, not including the perpetrator, counted at least 232 mass shootings as of May 26. (The archive, a nonprofit organization, has counted 15 mass murders, which it defines as four or more people killed, in 2021.)

There is little consensus on the definition of a mass shooting, complicating the efforts of nonprofits and news organizations to document the scope of the problem.

The Violence Project follows the narrow definition of the Congressional Research Service, requiring the attacks to be in public and excluding domestic shootings and those “attributable to underlying criminal activity.” CNN has defined a mass shooting as one with four or more injuries or deaths. The Washington Post’s effort to track public mass shootings includes shootings with four or more people killed, but does not include robberies or domestic shootings in private homes.

This year they have included shootings in Atlanta. Indianapolis. Boulder, Colo. Boone, N.C. And now San Jose.

To some, it might have seemed as if mass shootings all but halted during the coronavirus pandemic, with a year passing between large-scale shootings in public places. But the shootings never stopped. They just weren’t as public.

The Gun Violence Archive counted more than 600 such shootings in 2020, compared with 417 in 2019.