The two pistols came off the cops he shot...
Seems odd to me - he entered a secure, locked-down facility with a shotgun and managed to disarm two guards?
Perhaps he knew the habits of the guards well, although the media said he used a fake ID to enter the building, which means may not have had much opportunity to study his target.
Anyway, seems like Alexis either counted on having quite a bit of luck, or we aren’t being told something.