According to the U.S. Bureau of Labor, nearly one-quarter of workplace shootings occur because the person was fired or laid off.
The deadliest shooting took place 20 years ago at a South Bay tech company. A disgruntled employee opened fire at ESL Incorporated in February 1988 after becoming obsessed with a female colleague.
Richard Farley had also been laid off shortly before the shooting. Farley entered ESL heavily armed, killed seven co-workers and injured four others.
Farley is currently on death row at San Quentin.
Bill Lee, who worked at ESL at the time of the shooting, helped rescue some of his co-workers and wrote about the incident in his book "Chinese Playground: A Memoir."
"When the gunfire occurred, I was confused," Lee said. "Part of me sounded familiar to this because I grew up on the street...when this occurred happened, it was familiar to me but at the same time I didn't want to believe it was happening."
Another workplace shooting took place at Siport, a chip making company in Santa Clara. In November 2008, a test engineer killed three company executives after being laid off. The gunman was arrested the following day and is also currently in prison.
"We now have enough studies and validation that there is a continuum," said Garry Mathiason, a labor attorney who studies workplace violence. "Sometimes, the very slow continuum might last years. Sometimes, it's very fast -- three or four weeks of pressure."
According to the U.S. Bureau of Labor Statistics:
-- 86 percent of deaths were in the private sector
-- 43 percent of workplace homicides involve current employees
-- 24 percent of shooters had been fired or laid off.
Mathiason says intervention has proven to work. The U.S. Postal Service had a series of workplace shootings in the 1980s, but managed to turn things around.
"They adopted preventative services, managerial training, policies (and) processes" Mathiason said.
Those preventative measures are being adopted in other companies.