WASHINGTON — The deadly attack at the Washington Navy Yard, this week, was carried out by one of the military’s own: A defense contract employee and former Navy reservist, who used a valid pass to get onto the installation and started firing inside a building, killing 12 people, before he was slain in a gun battle with police.

The motive for the mass shooting — the deadliest on a military installation in the U.S. since the tragedy at Fort Hood, Texas, in 2009 — was a mystery, investigators said. But a profile of the lone gunman, 34-year-old Aaron Alexis, was coming into focus. He was described as a Buddhist, who also had flares of rage, complained about the Navy and being a victim of discrimination and had several run-ins with law enforcement, including two shootings.

U.S. law enforcement officials told The Associated Press that Alexis had been suffering a host of serious mental issues, including paranoia and a sleep disorder. He also had been hearing voices in his head, the officials said. Alexis 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.

The Navy had not declared him mentally unfit, which would have rescinded a security clearance Alexis had from his earlier time in the Navy Reserves.

However, Rhode Island police did warn the U.S. Navy last month that Alexis had reported “hearing voices,” raising further questions about how he gained security access at the complex where he went on a shooting rampage.

Family members told investigators Alexis was being treated for his mental issues.

Alexis.

Monday’s onslaught at a single building at the highly secure 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. Mayor Vincent Gray said there was no indication it was a terrorist attack, but he added that the possibility had not been ruled out.

“This is a horrific tragedy,” 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 mass shooting at a Newtown, Conn., elementary school that killed 20 students and six women. The weapon was also used in the shooting at a Colorado movie theater that killed 12 and wounded 70.

Top: Aaron Alexis. Bottom: A Washington, DC Metropolitan police officer (L) and a Naval District Washington policemen (R) stand guard at the main gate of the Washington Navy Yard in Washington, September 17, 2013. REUTERS/Jason Reed

For much of the day, 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.

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

Navy Yard employees returned to work Thursday, Sept. 19. 

Thursday was a regular work day at the Navy Yard, except at Building 197, where the shootings took place. It was closed, as was the gym, which is being used to stage FBI agents investigating Monday’s rampage, WRC-TV, Washington, reported.  

— TCSM, Reuters, UPI, TAAN