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, US media have reported.
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 attack.
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.
A Washington Nationals baseball game originally scheduled for Monday night was rescheduled to Tuesday afternoon - the baseball stadium is just blocks from the Navy Yard.
The flag flew at half-mast over the White House, where hours after the attack President Barack Obama mourned "yet another mass shooting".
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.
On Monday, however, White House Press Secretary Jay Carney deemed any talk of the political issue "inappropriate" so soon after the killings.
No motive is known for the attack.
The victims ranged in age from 46 to 73, and details about their identities and lives have just begun to emerge.
The shooting began about 8:20 local time (12:20 GMT) on Monday, when a gunman apparently 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.
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, you know, clearance to get... credentials to be able to get on the base," Washington DC Mayor Vincent Gray told CNN.
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, 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.
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 reportedly armed with an AR-15 semi-automatic rifle, a shotgun and a handgun that he took from a police officer at the scene.
Police initially responded to reports of two other armed suspects, but after exhaustive searches authorities said they were all but certain that Alexis was the only gunman.
Alexis was formerly a petty officer 3rd class who served full-time in the naval reserve from 2007-11, said the US Navy.Washington Police Chief Cathy Lanier told reporters the gunman "was engaged in shooting with police officers" when he died. "There is no question he would have kept shooting," she added.
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. It was not immediately clear why he left the military, but unnamed military officers told US media there had been a series of issues during his service.
According to media reports, Alexis was a Buddhist convert who had had two previous gun-related brushes with the law.
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.In 2004 he was arrested in Seattle for shooting a construction worker's car tyres, in what he described as an enraged "black-out".
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.
