The Northern California transit employee who fatally shot nine of his coworkers this week was also under investigation for racist remarks, according to local media reports.
Authorities say 57-year-old Samuel Cassidy opened fire at his workplace, a light rail yard in San Jose, on Wednesday.
Cassidy shot himself minutes later as police arriving on the scene closed in, according to the county sheriff.
The sheriff said Friday Cassidy had accumulated a dozen firearms, Molotov cocktails, and some 22,000 rounds of ammunition at his home - before setting it on fire.
She added that Cassidy was known to be a "highly disgruntled" employee for many years, which may have motivated the attack.
But any attempt to pin down a specific motive may be complicated by the blaze at Cassidy's home, which destroyed much of the available evidence.
Local NBC affiliate KNTV, citing unnamed law enforcement sources, reported that Cassidy had recently been accused of making racist remarks at work, and faced a disciplinary hearing the day of the shooting.
But the news outlet later rolled back on its report, after the county's Valley Transportation Authority issued a statement denying reports of any planned meeting.
The transit agency said it was reviewing Cassidy's alleged history of "fearful or uneasy" remarks.
Cassidy's ex-wife, Cecilia Nelms, also recounted to local media that he would rant about his coworkers and bosses and sometimes lash out at her.
The Wall Street Journal, citing a Department of Homeland Security memo, reported that U.S. customs and border officers detained Cassidy in 2016.
They found in his possession "books about terrorism" and a memo book filled with notes expressing hatred of his workplace.
Wednesday's killings are just the latest in a string of mass shootings across the U.S. over the past three months.