A transit employee who fatally shot 9 co-workers and killed himself at a Northern California rail yard was “highly disgruntled” long before carrying out the latest U.S. shooting rampage, the county sheriff said on Thursday.

Local authorities have declined to speculate on a precise motive for the gun violence that flare up on Wednesday in San Jose, California, noting that their work at the scene could take days, assisted by the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The gunman’s feelings about his workplace at the Santa Clara Valley Transportation Authority (TVA) “may have contributed to why he targeted” his co-workers, Sheriff Laurie Smith said in a written statement.

“Based on recent developments in the investigation we can say that the suspect has been a highly disgruntled VTA employee for many years,” Smith added.

The statement identified the gunman as 57-year-old Samuel Cassidy whose identification had been widely reported by media outlets since soon after the shooting.

The gunfire occurred just after a morning union meeting at a light-rail maintenance yard about 50 miles south of San Francisco in the heart of Silicon Valley. According to Smith, Cassidy shot himself minutes later as police arriving on the scene closed in.

Cassidy and the 9 victims gunned down were all employees of the Santa Clara VTA. The victims were found in 2 buildings on the site, near the city’s airport.

U.S. customs and border officers arrested Cassidy in 2016 as he returned from the Philippines and found that he professed a hatred of his workplace, the Wall Street Journal reported, quoting a Department of Homeland Security memo.

At the time Cassidy had in his possession “books about terrorism and fear and manifestos … as well as a black memo book filled with lots of notes about how he hates the VTA,” the Journal cited the memo as stating.