WASHINGTON (AP) - A defense contractor's deadly shooting rampage at the Washington Navy Yard is raising questions about the adequacy of the background checks done on contract employees who hold security clearances. Navy and White House security reviews have begun as a picture emerges of the gunman as an agitated and erratic figure whose behavior and mental state had repeatedly come to authorities' attention but didn't seem to affect his security clearance. Law enforcement officials say they've found no manifesto or other writings that would suggest a political or religious motivation for Aaron Alexis. However, they say he did suffer from serious mental health problems, including paranoia and a sleep disorder. The Navy says Alexis obtained his security clearance when he enlisted in the Navy reserves in 2007, which was good for 10 years and remained valid after he left the service in 2011. During his time of active service, the Navy says Alexis had incidents of insubordination, disorderly conduct and being absent from work without authorization, but nothing that gave commanders any clue he was capable of such violence. The 34-year-old shot to death 12 people before being killed by police in a shootout that lasted more than 30 minutes.