James Wesley Howell was arrested early Sunday in Santa Monica, California, with the weapons and explosives in a car he apparently drove from Indiana.
He told police he was headed to a gay pride event in West Hollywood that attracts hundreds of thousands of people.
It is unclear whether Mr Howell intended any violence at the LA Pride event, but the timing of the arrest — hours after the massacre at a gay nightclub in Orlando, Florida — put police and event organisers on heightened alert.
An Indiana probation officer met with Mr Howell, 20, of Charlestown, Indiana, three weeks ago, rated him a low-level offender, and had yet to schedule the in-home visit, said James Hayden, chief probation officer in Clark County.
Mr Howell did not have permission to leave Indiana after pleading guilty to a misdemeanour intimidation charge. Authorities there are seeking to have him returned as a probation violator. The FBI is continuing to investigate.
“We have processed the vehicle and are consulting with prosecutors to determine if charges apply,” said Laura Eimiller, an FBI spokeswoman in Los Angeles.
Court records in Indiana and friends depict Mr Howell as a gun enthusiast with a quick temper.
Twice within four days last October, he was accused of pulling a gun and making threats.
The first incident involved Mr Howell’s then-boyfriend and the second a neighbour, identified in police records as Jeremy Hebert.
Mr Howell was charged with intimidation in the case involving Mr Hebert, a conviction that led to his year-long probation and weapons prohibition.
Mr Hebert remembered Howell being “hot-headed” but said he had no interest in pursuing an intimidation charge against him.
“I wasn’t going to ruin his life for it,” Mr Hebert said, adding that he was fine as long as Mr Howell did n0t return to the neighbourhood.
Prosecutors apparently pursued the case without Mr Hebert.
Grace Logsdon, Mr Howell’s former roommate, said Mr Howell owned five guns and liked to spend time at a shooting range. He enjoyed teaching Ms Logsdon about guns, she said, but he had a bad temper.
During one trip to a gun range, she said Mr Howell told her: “I wish I could kill a lot of people.”
She said Mr Howell lived in a duplex apartment with Ms Logsdon, her husband and Mr Howell’s now-former boyfriend late last year.