Investigators say Alexis had a valid pass to enter the secure building - but have not discovered a motive
The ex-US Navy reservist who killed 12 people and wounded eight at a Washington DC Navy installation had received treatment for mental health issues. Aaron Alexis, 34, was treated for paranoia, trouble sleeping and hearing voices, the Associated Press reported.
A contractor for the Navy, he had a valid pass for the secure site at the Washington Navy Yard, authorities said. Alexis was shot and killed by police during the shooting.
On Tuesday, the US capital remained in a state of shock and mourning. Defence Secretary Chuck Hagel laid a wreath at US Navy Memorial Plaza in honour of the Navy Yard victims. The flag flew at half-mast over the White House, where hours after the attack President Barack Obama mourned "yet another mass shooting".
Secretary of the Navy Ray Mabus has ordered a physical security review of all Navy and Marine Corps installations. A senior Democrat in the US House of Representatives said he expected the incident would reignite the nation's long-standing gun control debate.
"I'm sure that it will renew the discussions about access to weapons that can be used to kill a lot of people quickly," said Steny Hoyer. White House spokesman Jay Carney said Mr Obama would continue to push for tighter gun control laws in the wake of Monday's shooting.
The shooting began about 8:20 on Monday, when a gunman shot his way into the heavily guarded Building 197, headquarters for Naval Sea Systems Command, which builds and maintains ships and submarines for the Navy. Witnesses said the gunman sprayed bullets in a hallway and fired from a balcony down on to workers in an atrium at the heavily secured installation in the US capital. He was armed with a shotgun legally purchased in Virginia, as well as one handgun authorities say may have been obtained inside the naval installation.
The attack only ended when police stormed the building and shot him dead, police said.
"There is no question he would have kept shooting," Washington Police Chief Cathy Lanier told reporters on Monday. On Tuesday, authorities said they believed Alexis had acted alone and that all victims - ranging in age from 46 to 73 - had been identified.
Authorities have revealed no motive for the attack, and have begun combing the US for hints about Alexis' past. As an employee of an IT contractor, Alexis apparently had a card granting him access to the building - even though he had had several run-ins with the law and had been discharged from the Navy under a cloud, authorities said.
"It really is hard to believe that someone with a record as chequered as this man could conceivably get clearance to get credentials to be able to get on the base," Washington DC Mayor
Vincent Gray told CNN.
Anthony Little says his brother-in-law Aaron Alexis had "shown no sign" that he could carry out a shooting.
Alexis has this in common with most mass killers. They rarely show any overt signs of impending homicide
Alexis left the Navy as a petty officer 3rd class, after serving full-time in the naval reserve from 2007-11. He received two routine medals for his service in the Navy: the National Defense Service Medal and the Global War on Terrorism Service Medal.
Alexis separated from the Navy reserves under what is called a general discharge, a status that suggests misconduct. He had been cited for misconduct - including insubordination, disorderly conduct and excessive absences from work - at least eight times during his Navy career, the Washington Post reported on Tuesday.
According to media reports, Alexis was a Buddhist convert who had had two previous gun-related brushes with the law. In 2004 he was arrested in Seattle for shooting a construction worker's car tires, in what he described as an enraged "black-out". And Alexis was questioned by Fort Worth police in 2010 after firing his gun through the floor of an upstairs apartment. He said it was an accidental discharge. Also, in August 2008 he was arrested for disorderly conduct in the state of Georgia, according to the Smoking Gun, a website that collects police and court documents. Details on the incident were not available.
On Tuesday, Alexis' employer said the military should have made his record known.
"Anything that suggests criminal problems or mental health issues, that would be a flag. We would not have hired him," Thomas Hoshko, chief executive officer of The Experts ( government sub contractor), told the Washington Post.
The IT contractor confirmed Alexis had worked since July at six different military installations and had only been at the Navy Yard for a few days before the shooting.
The assumption that another shocking mass murder will make people more amenable to gun control is not realistic. The debate is so emotional, so partisan that the two sides are already in two hostile camps. They are taking their familiar stances and reacting in their typical fashion....as they do with each mass killing. It would take something far more catastrophic for many Americans to relent on gun control. Or perhaps, going in a gentler direction, a visitation from an angel might point out that lethal weapons beget violence and death.
- Born in New York City in 1979; last known address was Fort Worth, Texas
- IT worker for defense department subcontractor The Experts
- Full-time US Navy reservist from 2007 till discharge in 2011, ranked petty officer 3rd class
- Awarded National Defense Service medal and Global War on Terrorism Service medal
- Said to have been Buddhist convert, had gun-related brushes with law in 2004 and 2010