New details are emerging about Aaron Alexis, the Navy contractor who killed 12 people at the Washington, D.C. Navy Yard on Monday morning. He reportedly suffered from a series of serious mental issues, including paranoia and a sleep disorder, Mayor Vincent Gray confirmed on Fox and Friends Tuesday.

Alexis had been hearing voices in his head and had been treated since August by the Veterans Administration for his mental problems, the officials said. They spoke on condition of anonymity because the criminal investigation in the case was continuing.

Alexis, 34, was discharged from the Navy two years ago after serving hitches in Texas and Illinois. 

He sprayed bullets from the fourth floor down to the cafeteria area in the Naval Sea Systems Command headquarters in southeast Washington in an attack that began just after 8 a.m. Alexis died later as he traded shots with responding police, although it was unclear if he killed himself or was brought down by cops.

At a late night news conference Monday, Metropolitan Police Chief Cathy Lanier said she was confident that Alexis was the "single and sole person responsible" despite earlier reports of a possible second shooter.