WASHINGTON (AP) — The former Navy reservist who killed 12 people at the Washington Navy Yard had been hearing voices and was undergoing treatment in the weeks before the shooting rampage, but was not stripped of his security clearance, officials said Tuesday.Aaron Alexis, a 34-year-old information technology employee with a defense contractor, used a valid pass to get into the highly secured installation Monday morning and started firing inside a building, the FBI said. He was killed in a gun battle with police.The motive for the mass shooting — the deadliest on a military installation in the U.S. since the attack at Fort Hood, Texas, in 2009 — was a mystery, investigators said.U.S. law enforcement officials told The Associated Press there was no known connection to terrorism and investigators have found no manifesto or other writings suggesting a political or religious motive.Alexis had been suffering a host of serious mental problems, including paranoia and a sleep disorder, and had been hearing voices in his head, according to the officials, who spoke on condition of anonymity because the criminal investigation was still going on.He had been treated since August by Veterans Affairs, the officials said.The Navy had not declared him mentally unfit, which would have rescinded a security clearance Alexis had from his earlier time in the Navy Reserve.The assault is likely to raise more questions about the adequacy of the background checks done on contract employees and others who are issued security clearances — an issue that came up most recently with National Security Agency leaker Edward Snowden, an IT employee with a government contractor.In the hours after the Navy Yard attack, a profile of Alexis began coming into focus.A Buddhist convert who had also had flare-ups of rage, Alexis, a black man who grew up in New York City and whose last known address was in Fort Worth, Texas, complained about the Navy and being a victim of discrimination. He also had run-ins with the law over shootings in 2004 and 2010 in Texas and Seattle, and was ticketed for disorderly conduct after being thrown out of a metro Atlanta nightclub in 2008.Alexis’ bouts of insubordination, disorderly conduct and being absent from work without authorization prompted the Navy to grant him an early — but honorable — discharge in 2011 after nearly four years as a full-time reservist, authorities said.