Washington (AFP) - A disgruntled, heavily-armed employee shows up at work and murders nine of his colleagues: this week's carnage at a California rail yard is every American employer's worst nightmare come true.

Samuel Cassidy, a 57-year-old white man, fired around 40 shots Wednesday at the public transport rail yard where he worked as a maintenance man in San Jose, California.

He was not the first US employee to commit mass murder at work, and in a country awash with guns he is unlikely to be the last. 

From 1970 to 1990, employees or ex-employees of the US Postal System alone killed around 40 people in a series of shooting at mail facilities. Americans invented the term "going postal" to describe spasms of violence in the workplace.

The coronavirus pandemic provided a pause in this violence. But as many companies reopened, with this came a return of shootings. In April an employee at a furniture store in Texas shot and killed one person, and a former employee killed eight people at a FedEx facility in Indianapolis, Indiana. 

When police arrived at the rail yard on Wednesday, Cassidy killed himself. Three semi-automatic pistols and 32 magazines were found on him.

"He was very deliberate, very fast; he knew where employees would be," Sheriff Laurie Smith told NBC television. 

A search this week of Cassidy's home -- which was set ablaze shortly before the attack -- discovered 12 guns, some 22,000 rounds of ammunition, and suspected Molotov cocktails, Smith said in a statement Friday.

His motive has yet to be determined.

But Cassidy "has been described by fellow co-workers as being disgruntled," and detectives are investigating his relationship with his employers and co-workers, said Smith.

He may have chosen his targets carefully. He reportedly told one co-worker, "I'm not going to shoot you."

Cassidy's ex-wife said that during their marriage, which ended in 2005, he complained a lot about his job.

"He just thought that some people got more easy-going things at work, and he'd get the harder jobs," Cecilia Nelms told the Mercury News, a San Jose daily, as she described a man with violent mood swings.

When he returned from a trip to the Philippines in 2016 Cassidy's luggage was searched. Customs agents found "books on terrorism" and notes about how much he hated the company he worked for, according to a memo from the Department of Homeland Security (DHS) that was revealed by the Wall Street Journal.

A spokeswoman for the agency declined to comment on the case saying the probe was under way. She highlighted the department's efforts to identify "behavioral indicators associated with targeted violence and policy to improve information sharing with our partners."

"You can't identify people in advance, even if they say I want to kill that guy," said James Alan Fox, a criminologist at Northeastern University in Boston and expert on violence in the workplace.

He said that in companies "there are people out there who are worrisome. But the vast majority of them will never do anything in response to the grievances, other than complain."

Grievance

What is more, Fox said, shootings in the work place are relatively rare if you consider the size of the workforce in America and the many millions of guns in circulation in the country. A third of all adults say they own at least one firearm.

In 2017 458 people died in workplace shootings, according to the latest official statistics, but only 77 of these fatalities were committed by workers. The others were done by burglars, relatives of employees or disgruntled customers.

In America companies are legally bound to provide a safe environment for people to work in. So they are more and more conscious of the risk of violence and take steps to deal with this danger.

Around 45 percent of American workers say their company has a workplace violence prevention policy, according to the Society for Human Resources Management (SHRM).

The organization gives advice to companies, ranging from identifying risk situations to teaching people how to flee or hide, say, in the event of a workplace shooting.

"We've learned certain lessons over the years," said Fox.

"A company can do everything they should be doing. But at the end of the day, even if they're treating all their employees fairly, it doesn't mean that all their employees will think that they are treated fairly.

As guns are readily available in America, said Fox, "sometimes these employees with a grievance have a weapon to express their displeasure."