All 12 victims, aged 46 to 73, were civilians caught up in the shooting spree on Monday morning by the former Navy reservist, who received security clearance to work as an information technology contractor at the site despite a history of misconduct and signs of mental illness.

Alexis, 34, was killed in a gun battle with police officers.

With his motive still a mystery, US Defence Secretary Chuck Hagel acknowledged there were “red flags” before Alexis received security clearance.

“Obviously when you go back in hindsight and look at all this, there were some red flags - of course there were,” Hagel told a news conference. “And should we have picked them up? Why didn’t we? How could we have? All those questions need to be answered.”

The US Department of Veterans Affairs said he twice visited VA emergency rooms for insomnia but never sought care from a mental health specialist and denied he suffered from anxiety or depression.

Alexis received medication for insomnia in Providence, Rhode Island, on August 23 and Washington on August 28.

“On both occasions, Mr. Alexis was alert and oriented, and was asked by VA doctors if he was struggling with anxiety or depression, or had thoughts about harming himself or others, which he denied,” the VA said in a statement.

A few weeks earlier, on August 7, Alexis told police while on a business trip to Newport, Rhode Island, that he had trouble sleeping because he was “hearing voices.” He believed people were following him and “sending vibrations into his body,” according to a Newport police report.

Newport police found his behaviour so odd that they called Navy police to alert them and sent them a copy of their report.

The Navy said it was looking into the Newport incident.

Alexis, who was a resident of Fort Worth, Texas, legally bought the 12-gauge Remington shotgun used in the rampage for $419 at the Sharpshooters Indoor Shooting Range and Pro Shop in Lorton, Virginia on Saturday, the store’s lawyer, J. Michael Slocum said on Wednesday.

Slocum said the store refused to sell Alexis a handgun.

“He asked about buying a handgun but was told, no he couldn’t buy a handgun because he’s not a Virginia resident. At that point, he bought the shotgun,” Slocum said.

He said Alexis rented an AR-15 assault rifle and fired it at a store shooting range, and then bought the shotgun along with two boxes of ammunition. According to store employees, he did not ask to buy an AR-15, Slocum said.

Alexis brought a shotgun into the Naval Sea Systems Command building and grabbed at least one handgun once inside, investigators said, opening fire from the fourth floor atrium on helpless victims in a cafeteria below.

Investigators have a “working theory” that Alexis arrived at the building with the shotgun barrel removed, entered a men’s room with his bag or backpack, and assembled it, according to a source close to the investigation.

The shooting was the worst at a military facility since 2009 when 13 people were killed and 31 wounded at Fort Hood, Texas, by an army psychiatrist who later said he carried out the shootings because of US wars in Muslim countries.

Alexis had been arrested twice, once in 2004 and again in 2010, for illegally discharging firearms, though in each case charges were not pursued.