9:55:25: Be advised, we have multiple weapons. One rifle and a shotgun
There was only one shotgun and it was the one found in the vehicle per police reports.
More than likely they searched his vehicle and the trunk and found that shotgun before going into the school. A mother was interviewed by the media who was there at the front door at that time while there was still shooting in the school and saw the doors of the vehicle open and stuff from inside the vehicle strewn about outside the open doors. Odds are they popped the trunk and found the shotgun then.