The key here "was in possession". According to the Seattle Times, the gun and "unregistered ammo" were in his car.
While the Secret Service detained Murray, officers from Washington’s Metropolitan Police Department tracked down his car, where they found a rifle and ammunition. The Secret Service then turned Murray over to the police.
It wasn't like he was setting up outside of the "official residence" for a shooting.
Also worth noting, Kneepads wasn't at the residence and hasn't even moved into the Naval Observatory in the first place. Not to mention this guy was a nutjob...
An intelligence bulletin from Texas had warned law enforcement about Murray, officials said. Murray’s mother had also warned the police that her son was in Washington, authorities said.
A 31 year old dude, Paul Murray, already known to law enforcement, and his mom dropped a dime on him. I am sure he is mentally stable. I am also sure this will be used to justify keeping the National Guard in D.C.
We are getting closer, this guy had the right weapon but clearly was off his meds....