The former Navy reservist who slaughtered 12 people at the Washington Navy Yard had been hearing voices and was being treated for mental problems 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.

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U.S. law enforcement officials told The Associated Press that there was no known connection to international or domestic terrorism and that investigators have found no manifesto or other writings suggesting a political or religious motivation.

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 for his mental problems, 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The Navy had not declared him mentally unfit, which would have rescinded a security clearance Alexis had from his earlier time in the Navy Reserves.

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. During his service, he repaired aircraft electrical systems at Fort Worth.

In addition to those killed at the Navy Yard attack, eight people were hurt, including three who were shot and wounded, authorities. Those three were a police officer and two female civilians, authorities said. They were all expected to survive.

Monday's onslaught at a single building at the Navy Yard unfolded about 8:20 a.m. in the heart of the nation's capital, less than four miles from the White House and two miles from the Capitol. It put all of Washington on edge.

"This is a horrific tragedy," Mayor Vincent Gray said.

Alexis carried three weapons: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene, according to two federal law enforcement officials who spoke on condition of anonymity because they were not authorized to discuss the investigation.

The AR-15 is the same type of rifle used in last year's shooting at a Newtown, Conn., elementary school that killed 20 children and six adults. The weapon was also used in the shooting at a Colorado movie theater that killed 12 and wounded 70.

For much of the day Monday, authorities said they were looking for a possible second attacker who may have been disguised in an olive-drab military-style uniform. But by late Monday night, they said they were convinced the shooting was the work of a lone gunman, and the lockdown around the area was eased.

"We do now feel comfortable that we have the single and sole person responsible for the loss of life inside the base today," Washington Police Chief Cathy Lanier said.

President Barack Obama lamented yet another mass shooting in the U.S. that he said took the lives of American "patriots." He promised to make sure "whoever carried out this cowardly act is held responsible."

The FBI took charge of the investigation.

The attack came four years after Army psychiatrist Maj. Nidal Hasan killed 13 people at Fort Hood in what he said was an effort to save the lives of Muslims overseas. He was convicted last month and sentenced to death.

The dead in the Navy Yard attack ranged in age from 46 to 73, according to the mayor. A number of the victims were civilian employees and contractors, rather than active-duty military personnel, the police chief said.

At the time of the rampage, Alexis was an employee with The Experts, a company that was a Defense Department subcontractor on a Navy-Marine Corps computer project, authorities said.

Valerie Parlave, head of the FBI's field office in Washington, said Alexis had access to the Navy Yard as a defense contractor and used a valid pass.

The Washington Navy Yard is a sprawling, 41-acre labyrinth of buildings and streets protected by armed guards and metal detectors, and employees have to show their IDs at doors and gates. More than 18,000 people work there.

The rampage took place at Building 197, the headquarters for Naval Sea Systems Command, which buys, builds and maintains ships and submarines. About 3,000 people work at headquarters, many of them civilians.

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Witnesses on Monday described a gunman opening fire from a fourth-floor overlook, aiming down on people on the main floor, which includes a glass-walled cafeteria. Others said a gunman fired at them in a third-floor hallway.

Patricia Ward, a logistics-management specialist, said she was in the cafeteria getting breakfast.

"It was three gunshots straight in a row — pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward said.