Washington - Investigators on Tuesday tried to piece together what led a former US Navy reservist to open fire at a Washington base, killing 12 people before being gunned down by police.
Police identified the gunman as Aaron Alexis of Fort Worth, Texas, who served in the Navy from 2007 to 2011 before becoming a defence subcontractor for computer giant Hewlett-Packard.
The Federal Bureau of Investigation appealed to the public for information on the 34-year-old, whose military service was marked by disciplinary problems and who had had previous run-ins with police for discharging firearms.
"No piece of information is too small. We are looking to learn everything we can about his recent movements, his contacts and his associates," said Valerie Parlave, assistant director of the FBI's Washington field office.
Washington mayor Vincent Gray said he was baffled that the shooter got the job he had.
"It's hard to believe that someone with a record as checkered as this man could get clearance, credentials, to get on the base," he told CNN.
"I just met with the commandant of the Washington Naval Yard last week. We know this is one of the most secure facilities in the nation. So how this could happen is beyond belief."
Asked about a possible motive, Gray declined to speculate. But he said mandatory spending cuts that hit the government and the military this year may have reduced security at the yard.
"We somehow skimped on what would be available for projects like this and then we put people at risk. Obviously 12 people have paid the ultimate price for whatever - you know, whatever was done to have this man on the base," Gray said.
US authorities said Alexis appears to have acted alone in carrying out the murderous shooting spree, which press reports said also injured 14 at Washington's Navy Yard.
"We do now feel comfortable that we have the single and sole person responsible for the loss of life inside of the base today," Police Chief Cathy Lanier said late on Monday.
US Defence Secretary Chuck Hagel was to lay a wreath on Tuesday at the scene of the shooting to honour the victims.
The FBI released a photo of Alexis, who held the rank of an Aviation Electrician's Mate Third Class, and had served full-time in a logistics support squadron in Fort Worth, according to the Navy.
The shooting left Washington on edge and there was a security scare hours later at the White House when a man who apparently threw firecrackers over a fence at the US president's residence was swiftly arrested.
Alexis's motivation for opening fire, reportedly with an AR-15 assault rifle, was unclear. His four-year stint in the Navy was troubled, officers said.
"There is definitely a pattern of misconduct during his service," a US military officer, who spoke on condition of anonymity, told AFP.
Friends in Texas told US media that Alexis had an interest in Buddhism and was conversant in the Thai language and had thought about moving to Asia.
Most recently, Alexis was employed as an IT subcontractor for a company called "The Experts," which was working on a Hewlett-Packard contract to upgrade equipment for an intranet network used by the US Marine Corps and Navy, HP said in a statement.
A US defence official told AFP that the gunman had gained access to the Navy Yard "through legitimate means".
But it was unclear whether the military or HP had been aware of Alexis' brushes with the law, including two shooting incidents, before he was hired for the IT job.
The shooting sparked a massive show of force as police and federal agents descended on the Navy Yard, which is located on the Anacostia River, less than 3km from the Capitol.
President Barack Obama ordered that flags be flown at half mast in the US capital until Friday as a mark of respect for the dead.
Obama called the shooting a "cowardly act" and said that America was confronting "yet another mass shooting," saying troops in the military should not have to face danger at home.
One employee at the Navy Yard, Patricia Ward, said she had just paid for her breakfast at a cafeteria when shots rang out at about 08:20 (13:20 GMT).
"I was waiting for my friend to pay when we heard the gun shot. It was three gun shots straight in a row, 'pow-pow-pow,'" she told reporters.
"Three seconds later it was 'pow-pow-pow.' So it was like a total of seven gun shots. And we just started running."
Flights out of the nearby Reagan National Airport were briefly delayed local schools were on lockdown until anxious parents came to pick up their children in the afternoon.
The US Senate adjourned for the day as a precaution, and Washington's baseball team, the Nationals, whose stadium is adjacent to the Navy Yard, called off its Monday evening game.
Congressional activities were to resume as normal on Tuesday.
About 3 000 people work at the naval facility, which dates back to the early 1800s and includes a naval history museum.