Washington, D.C. - The deadly attack at the Washington Navy Yard 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.
Law enforcement officials say gunman Aaron Alexis carried three weapons at the Navy Yard: an AR-15 assault rifle, a shotgun and a handgun he took from an officer at the scene.
The shooting was the deadliest on a military installation in the U.S. since the tragedy at Fort Hood, Texas, in 2009.
U.S. law enforcement officials are telling 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 The Navy had not declared him mentally unfit, which would have rescinded a security clearance that Alexis had from his earlier time in the Navy Reserves.
Family members told investigators that Alexis was being treated for his mental issues.
While some neighbors and acquaintances described him as "nice," his father once told detectives in Seattle his son had anger management problems related to post-traumatic stress brought on by the terrorist attacks of Sept. 11, 2001. He also complained about the Navy and being a victim of discrimination.
The U.S. Navy said Alexis' rampage started with three shots fired around 8:20 a.m. Monday at the Naval Sea Systems Command Headquarters building, where about three thousand people work.
President Barack Obama is mourning what he called "yet another mass shooting" in the United States that he says took the life of American patriots.
He said the victims were "courageous Americans" who knew about the risks of serving overseas, but wouldn't have expected such "unimaginable violence" at home.
Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget.
It builds, buys and maintains the Navy's ships and submarines and their combat systems.