Washington Naval Yard shooter
What we know:
First, he didn't have an AR-15, doubtless to the great disappointment of certain legislators. Looks like he had a Remington 870, and two handguns he took off security personnel that he shot. The media are making gallant tries to explain why they were all claiming the AR-15. I suspect someone, somewhere, got that idea and everyone else repeated it.
Second, in 2004, he was in Seattle arrested after shooting out a parked car's tires, and nothing was done beyond the arrest.
Third, in 2010, he shot a bullet up into the apartment of a neighbor with whom he'd argued. He claimed he was cleaning the gun while cooking (?) and accidentally fired it. He was arrested but prosecutors declined to proceed.
Fourth, he was reportedly being treated for paranoid psychosis, which poses quite a risk of danger.
A couple of links that ask the right questions: