U.S. law enforcement officials say Aaron Alexis had been treated since August by the Veterans Administration for mental problems. Those officials say Alexis suffered from a host of mental issues including paranoia anda sleep disorder. He aslo had been hearing voices in his head.
They spoke on condition of anonymity because the criminal investigation in the case was continuing. The Navy had not declared him mentally unfit, which would have rescinded a security clearance that Alexis had from his earlier time in the Navy Reserves.
Family members told investigators that Alexis was being treated for his mental issues.
Washington D.C. Police Chief Cathy Lanier says authorities believe there was only a lone gunman responsible for the Navy Yard shooting rampage that left 13 people dead.
Authorities had earlier said they were searching for a possible second suspect.
Mayor Vincent Gray says the 12 shooting victims range in age from 46 to 73 years old. The gunman, identified as 34-year-old Aaron Alexis, was also killed.
Gray said at a news conference late Monday night that they are still notifying the families of about half of the 12 people who were killed.
The FBI says the gunman used a valid pass to get on to the Navy Yard.