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Why did the gunman kill 9 co-workers at the San Jose VTA light rail yard before killing himself?

Santa Clara County Sheriff's detectives, along with agents from the ATF and FBI, on Thursday started their first full day of investigating why a 57-year-old VTA employee would storm into a morning meeting and kill nine of his co-workers before killing himself in the most horrific mass shooting in the Bay Area in recent memory. 

Details began to emerge Thursday that Cassidy had expressed hatred about his job and colleagues going back years. In one case, federal officials had even become aware of Cassidy's views, according to a Wall Street Journal report.

The investigation will take time because the VTA light rail yard on Younger Avenue is so large and because gunman Sam James Cassidy killed his co-workers in two large areas of the yard, according to the sheriff's office. 

In addition, the investigation stalled while a bomb-sniffing dog picked up the scent of something suspicious. Detectives had to wait as the county bomb squad team went in to make sure the facility was safe, scouring it for explosives, inch by inch. Explosives were not found, but materials that could be used to make bombs were found in Cassidy's locker, authorities said.

However, some new details were released on Thursday morning.

SEE ALSO: San Jose VTA shooting kills 10, including employee who opened fire

Santa Clara County Sheriff Laurie Smith said that Cassidy had two semi-automatic weapons and 11 magazines. When authorities confronted him after the 6:30 a.m. Wednesday shooting, he killed himself.

Smith said her detectives are searching Cassidy's emails and texts, to try and figure out a motive.

"Those kinds of things may lead to something," Smith said, adding that so far, Cassidy had left "no written documents found that I'm aware of." 

She said that one of her employees had a relative who died in the massacre. 

SEE ALSO: Santa Clara County coroner identifies VTA shooting victims

Newly released surveillance video shows Cassidy as he walks through the VTA maintenance yard in San Jose where he shot and killed nine of his coworkers Wednesday. 

The footage released by Santa Clara County Sheriff's Office shows Cassidy calmly walking through the light rail yard the morning of the killings. According to the sheriff, it shows Cassidy after he had already opened fire at one rail yard building and before he entered a second building to continue shooting.

While a motive for the shooting has not been publicly revealed, his ex-wife told the Associated Press that he had talked about killing people at work more than a decade ago. 

"I never believed him, and it never happened. Until now," a tearful Cecilia Nelms told The Associated Press on Wednesday following the 6:30 a.m. attack at a light rail facility for the Valley Transportation Authority.

Nelms, who separated from Cassidy in 2004 and ultimately divorced him in 2009, said he used to come home from work resentful and angry over what he perceived as unfair assignments.

"He could dwell on things," she said. 

The two hadn't talked since. 

Federal authorities also took note of Cassidy when he returned to the US from a trip to the Philippines in 2016.

Cassidy carried "books about terrorism" and "a black memo filled with lots of notes about how he hates the VTA" when he was detained by US customs and border agents following the overseas trip, according to a Department of Homeland Security memo quoted by the Wall Street Journal.

Wednesday’s attack the deadliest in the San Francisco Bay Area.

In 1993, a gunman attacked law offices in San Francisco’s Financial District, killing eight people before taking his own life. But the VTA shooting claimed the lives of nine employees and the gunman himself. 

It also was Santa Clara County’s second mass shooting in less than two years. A gunman killed three people and then himself at the popular annual garlic festival in Gilroy in July 2019.

SEE ALSO: What we know about the VTA light rail gunman 

SEE ALSO: Widow of slain VTA employee says her husband and 'rock' died in her arms

The VTA light rail massacre was the 15th mass killing in the nation this year.

And it weighed heavily on the transit community across the nation.

VTA Acting General Manager Evelynn Tran said at a Thursday news conference that speaking to the victims' families was "utterly heartwrenching," saying that she felt "utterly helpless" in the wake of such violence. 

She said she wanted to honor all those who died and take the time to mourn and grieve with their loved ones. 

All of these mass shootings claimed at least four lives each for a total of 86 deaths, according to a database compiled by The Associated Press, USA Today and Northeastern University.

At the White House, President Joe Biden ordered flags to be flown at half-staff and urged Congress to act on legislation to curb gun violence.

"Every life that is taken by a bullet pierces the soul of our nation. We can, and we must, do more," Biden said in a statement.

Gov. Gavin Newsom visited the site and then spoke emotionally about the country’s latest mass killing.

"There’s a numbness some of us are feeling about this. There’s a sameness to this," he said. "It begs the damn question of what the hell is going on in the United States of America?"

Vice President Kamala Harris, who is from the Bay Area, noted her close ties to San Jose Mayor Sam Liccardo and that she has relatives in the South Bay. 

"It is tragic," she said. 

The shooting took place in two buildings and killed employees who had been bus and light rail operators, mechanics, linemen and an assistant superintendent over the course of their careers. One had worked for the agency since 1999.

The Santa Clara County Office of the Medical Examiner-Coroner identified the victims as Paul Delacruz Megia, 42; Taptejdeep Singh, 36; Adrian Balleza, 29; Jose Dejesus Hernandez, 35; Timothy Michael Romo, 49; Michael Joseph Rudometkin, 40; Abdolvahab Alaghmandan, 63, Lars Kepler Lane, 63 and Alex Ward Fritch, 49.

Singh had worked as a light-rail train driver for eight or nine years and had a wife, two small children and many family members, said his cousin, Bagga Singh.

"We heard that he chose the people to shoot, but I don’t know why they choose him because he has nothing to do with him," he said.

San Jose City Councilman Raul Peralez said Rudometkin was a close friend.

"There are no words to describe the heartache we are feeling right now, especially for his family," he wrote on Facebook. "Eight families are feeling this same sense of loss tonight and our entire community is mourning as well."

In court documents, an ex-girlfriend described Cassidy as volatile and violent, with major mood swings because of bipolar disorder that became worse when he drank heavily.

Several times while he was drunk, Cassidy forced himself on her sexually despite her refusals, pinning her arms with his body weight, the woman alleged in a 2009 sworn statement filed after Cassidy had sought a restraining order against her. 

Cassidy had worked for Valley Transportation Authority since at least 2012, according to the public payroll and pension database Transparent California, first as a mechanic from 2012 to 2014, then as someone who maintained substations. His annual salary was slightly more than $100,000. 

Officials also were investigating a house fire that broke out on Angmar Court where Cassidy lived shortly before the shooting, authorities said. 

Public records show Cassidy owned the two-story home where firefighters responded after being notified by a passerby. Law enforcement officers cordoned off the area near the home and went in and out Wednesday. The ATF and the FBI were called in to assist as gasoline and ammunition were found inside the home. 

Realtor Doug Suh, who lives across the street, told KTVU he was scared of Cassidy.  He witnessed the fire set at Cassidy's home first thing in the morning. 

"I talked to him five times in five years," Suh said. "I say ‘hi,’ and he stared at me, dirty looking all the time. So I ignore him every time I see him." 

Security camera footage taken from Suh's home shows Cassidy leaving his home at 5:45 a.m. The video shows he was wearing a blue jumpsuit and entering his white pickup truck with a large, black duffel bag.

Suh said he and his wife were both scared of Cassidy. 

Once, Suh said he backed up into Cassidy's driveway. In response, Cassidy yelled at him to get off his property.

"'You step on my driveway, get out!" Suh recalls Cassidy saying to him. "After that, I haven't talked to him since then."

KTVU reporter Tom Vacar and the Associated Press contributed to this article.