Aaron Alexis, the man identified as yesterday's shooter at the Navy Yard in Washington, apparently was a civilian contractor who held a "secret" security clearance and was scheduled to start working at the Navy Yard. This same Reuters story also indicates that he had successfully passed a background check in July 2013 conducted by Defense Security Service.
Yet...Alexis had been arrested before: in 2004 in Seattle for shooting the tires of construction vehicles, in 2008 in Georgia for "disorderly conduct," and in 2010 in Fort Worth for a negligent discharge of a weapon. He also suffered from schizophrenia and a sleep disorder. He also felt that he was the victim of discrimination in relation to a contracting job in Japan--the same contractor he was to begin working for in Japan.
Notwithstanding the harping on his having an AR-15, that is not true. The FBI has confirmed that Alexis was armed with a shotgun and two handguns. He apparently was found next to an AR-15. However, surveillance video shows that he obtained the handguns from security guards which he killed. (See this story, which indicates that, according to the FBI, Alexis arrived carrying only the shotgun, which he had recently purchased).
Of course, we should remember that as a military facility, like Ft. Hood, it too was a gun free zone.