it could be a recoil operated automatic loading/firing such as John Browning design, and as applicable in the 1917 M2 which is the longest lasting continuously deployed heavy machine gun in the US arsenal.
it could be that this fool filed the sear down so as the hammer isn't captured, forcing a 'runaway gun', which is when the shooter has no control- you pull the trigger once and hold on until all ammo is expelled/exhausted.
whether it was a fully automatic or a semi- or a freakin' black powder muzzle loader, this freak of a human set forth to intentionally injure masses of people he didn't know or know anything about other than he wanted to kill or maim them....
and that shit is some kind of sad.... the hate someone has to have to engage someone who they know nothing about and isn't trying to harm them is substantial, and it's being fostered in this country actively, by anyone with a platform it seems.