The man who killed 12 people at a Washington DC Navy installation had received treatment for mental health issues, US media report.
Former US Navy reservist Aaron Alexis, 34, was treated for paranoia, hearing voices and sleeplessness, the Associated Press reported.
A military contractor, 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.
Investigators say Alexis had a valid pass to enter the
secure building - but have not discovered a motive,
Rajini Vaidyanathan reports
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 stadium is just blocks from the Navy Yard.
Secretary of the Navy Ray Mabus has ordered a physical security review of all Navy and Marine Corps installations, a Navy official told the BBC.
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.
At the White House, where flags are flying at half mast as a mark of respect for the victims, press secretary Jay Carney said President Barack Obama would continue to push for tighter gun control laws in the wake of Monday's shooting.
Legally purchased shotgun
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.
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.
Previous reports suggested Alexis had used a well-known semiautomatic rifle known as an AR-15 during the shooting. But officials told the Associated Press on Tuesday that although the weapon was found at the scene, it was neither carried nor used by the gunman.
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.
Searching for a motive, authorities have begun digging into 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, you know, clearance to get... credentials to be able to get on the base," Washington DC Mayor Vincent Gray told CNN.
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 tyres, in what he described as an enraged "black-out".
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, an IT contractor called The Experts, 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 firm, told the Washington Post.
The company 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.