This booking photo provided by the Fort Worth Police Department shows Aaron Alexis, arrested in September, 2010, on suspicion of discharging a firearm in the city limits. (AP)
WASHINGTON — Investigators still don't know why Aaron Alexis went on a bloody shooting spree Monday that left 12 people dead but clues are emerging that the military veteran had suffered from serious mental illness and could act quickly -- and violently -- when angered.
U.S. law enforcement officials told the Associated Press that Alexis, who died in a shootout with law enforcement officers, had paranoia and a sleep disorder and was "hearing voices" in his head.
The Associated Press, quoting unidentified sources, said Alexis had been treated by the U.S. Veterans Affairs for serious mental illness, including "hearing voices."
Family members also told investigators that Alexis was being treated for his mental issues. Law enforcement officials told the AP that he had been treated since August by the Veterans Affairs. The officials spoke on condition of anonymity because the criminal investigation was continuing.
Alexis had also had run-ins with police over two shootings incidents -- one in Fort Worth and one in Seattle-- but was never charged. He was honorably discharged in 2011 as an active-duty Navy reservist despite bouts of misconduct, including insubordination, disorderly conduct and absent from work without authorization.
Whatever prompted Alexis' rampage, he moved quickly and methodically to carry out the deadly assault. Law enforcement official said it wasn't apparent that Alexis was targeting specific people, but that he apparently planned his actions in advance.
Police have released the names of seven of the 12 killed: They were Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46; and Vishnu Pandit, 61. No hometowns were listed.
At least three people, including a city police officer, suffered non-fatal gunshot wounds inside Building 197 at the Naval Sea Systems Command headquarters. Hospital officials said all three were expected to recover. Authorities said five other people suffered minor non-gun injuries.
U.S. flags were lowered to half-staff at the Congress and White House.
A federal law enforcement official told USA TODAY that Alexis was armed with an AR-15, which is a light-weight semi-automatic rifle, as well as a shotgun and a handgun. The federal official requested anonymity because of the fluid nature of the investigation.
The official said Monday that Alexis, who had been staying at a nearby Residence Inn since late August or early September, legally purchased at least some of the weapons used in the assault very recently in Virginia.
The FBI said late Monday that Alexis had a valid pass and security clearance to enter the Navy Yard as a civilian contractor. Valerie Parlave, assistant director in charge of the FBI's Washington field office, said the shooter had "legitimate access to the Navy Yard.''
Alexis had been a full-time Navy reservist from 2007 to early 2011, leaving as a petty officer third class, the Navy said. He had been an aviation electrician's mate with a unit in Fort Worth.
From 2008 to 2011, Alexis served with Fleet Logistics Support Squadron 46 at the Naval Air Station in Fort Worth. He lived in that area and was arrested at least once in 2010 for firing a gun through the ceiling of his apartment. He told police it had been an accident.
Seattle police released details late Monday of another shooting incident in 2004 in which Alexis shot the rear tires of a vehicle owned by a construction worker doing work in his neighborhood. Alexis told police he had an anger-fueled "blackout," but added that he felt he had been "mocked" and "disrespected" by the workers.
Alexis also told police he was present during "the tragic events of September 11, 2001" and described "how those events had disturbed him." Detectives later spoke with Alexis' father in New York, who told police Alexis had anger-management problems associated with PTSD, and that he had been an active participant in rescue attempts on 9/11.
The Navy had not declared its defense contract employee mentally unfit, which would have rescinded a security clearance that Alexis had from his earlier time in the Navy Reserves.
The shooting caused pandemonium at the Navy complex that is located less than 3 miles from the White House.
Rick Mason, a program management analyst, said a gunman began shooting from a fourth-floor overlook in the hallway outside his office. He said the gunman was aiming down at people in the building's cafeteria on the first floor.
Patricia Ward, a logistics management specialist, said she was in the cafeteria. "I heard three shots — pow, pow, pow. Thirty seconds later I heard four more shots."
Then panic, as people tried to get out of the cafeteria. "A lot of people were just panicking. There were no screams or anything because we were in shock."
While Alexis' background raised troubling conditions about his mental state, close friends recall him as bright and friendly.
Nutpisit Suthamtewakul met Alexis around three years ago at a Fort Worth Buddhist temple and quickly became friends with him. Alexis moved in with Suthamtewakul, who owns the Happy Bowl Thai restaurant, and the two would have late-night drinking sessions, fueled by Heineken beer.
Suthamtewakul would move four times over the next three years. Alexis moved with him. Struggling to keep a job, Alexis didn't contribute to the rent but would come to Suthamtewakul's restaurant and help with deliveries for free, including to some of Fort Worth's roughest neighborhoods, Suthamtewakul said. Often, Alexis would have his .45-caliber pistol tucked in his belt, he said.
Alexis spent much of his time with Suthamtewakul and his Thai friends. He didn't have a temper but wasn't afraid to use his fists, he said. Once, a friend of Suthamtewakul's shoved Alexis over an argument about a girl. Alexis punched him in the nose, bloodying him and drawing the cops to the scene.
Suthamtewakul trusted and liked Alexis, even asking him to be his best man at his wedding in December. In July, with Suthamtewakul's new wife now living with the pair, Alexis moved out.
"He was a good guy to me," Suthamtewakul said. "I still can't believe he would do that."
Kevin McDonald, owner of Kevin's Hometown Furniture, next to the Happy Bowl restaurant, said he would often chat with Alexis outside the restaurant. Alexis would be taking a cigarette break outside the restaurant and McDonald would chastise him, telling him he should quit.
McDonald knew Alexis had been in the Navy and the two would talk politics and current events, ranging from the attack on the U.S. embassy in Benghazi to the latest news out of the White House, he said. Alexis didn't have heated opinions on any of the topics.
"Nothing out of the ordinary," McDonald said. "I probably made more negative comments on President Obama than he did."
Alexis would often show up to work in camouflage pants, military-style boots and a crisp white T-shirt, he said.
A few weeks ago, Alexis asked McDonald if he would buy his couch from him. He said he had landed a good job in Washington D.C. and either had to sell the couch or pay to store it.
"He made it seem like he was coming back," McDonald said. "The next day, he was gone."
Rick Jervis reported from Fort Worth; Kevin Johnson from Washington. Contributing: Associated Press