Santa Monica police and the FBI are examining the intentions of James Wesley Howell, 20, who told police he was going to LA Pride in West Hollywood, which annually draws hundreds of thousands of people.
His arrest came just hours after 50 people were shot and killed in a gay nightclub in Orlando, Florida, though police said they had found no evidence of a connection.
Mr Howell, of Jeffersonville, was arrested at 5am, after residents reported suspicious behaviour by a man who parked his white Acura sedan facing the wrong way. When officers arrived, they saw an assault rifle sitting in Mr Howell’s passenger seat, Santa Monica police Lt. Saul Rodriguez said.
They then found two more assault rifles, high-capacity magazines and ammunition, and a five-gallon bucket which contained chemicals that could be used to make an explosive device.
Santa Monica police chief Jacqueline Seabrooks initially tweeted that Mr Howell told officers he wanted to “harm” the gay pride event, but later corrected her statement to say only that Mr Howell said he was going to the parade.
A Facebook page of Mr Howell’s includes photos of the Acura. There is no enmity toward gay people or any notable political activism. One post says he is signing a petition to legalise marijuana.
The page’s most recent public post, from June 3, shows a photo comparing an Adolf Hitler quote to a Hillary Clinton one. An anti-Clinton, pro-Bernie Sanders photo was posted in February.
The site says Mr Howell worked as an auditor for a company that makes air filters.
Mr Howell was charged in October, in Clark County, Indiana, with pointing a firearm at someone, and with intimidation. He made a deal with prosecutors, in April, to plead guilty only to the misdemeanor intimidation charge. He was given a one-year prison sentence that the judge suspended in favour of strict probation against him having weapons.
A friend of Howell’s, Joseph Greeson, said Mr Howell’s parents had not seen him for days and that they called Mr Greeson’s parents looking for him. Mr Greeson said Mr Howell had no ill will towards gay people.