Wait when did they find the weapon?? Last I heard they didn't have the weapon and the 2 people they arrested were the wrong guy. So it sounds like the shooter didn't leave the gun behind, so I would expect them to only get the weapon if they got the shooter unless I missed something between yesterday and today.
Edit: Found the update that I missed from the FBI this morning, they apparently found the weapon in a "wooded area", so shooter still unknown