The FBI has identified the dead suspect as US military veteran Aaron Alexis (34) of Fort Worth, Texas, a navy contractor who had two gun-related brushes with the law. He was discharged from the navy reserve in 2011 after a series of misconduct issues, according to a navy official.
Alexis opened fire at a Washington navy yard on Monday in a burst of violence that killed 13 people, including the gunman, and set off waves of panic at the military installation just miles from the White House and US Capitol.
He was killed in one of several gun battles with police after he entered the Naval Sea Systems Command headquarters at about 8.20am (12.20pm GMT) and started picking off victims in a cafeteria from a fourth-floor atrium, witnesses said.
That set off pandemonium, with fire alarms sounding and security officers yelling at people to leave the building. Hundreds fled, some scrambling over walls to escape the gunfire. A loudspeaker announcement ordered those who remained to stay in their offices.
The motive remains unknown. Alexis was armed with an AR-15 military-style assault rifle, a double-barrelled shotgun and a handgun, a federal law enforcement source said.
Eight people were injured, including three who were shot, said Washington mayor Vincent Gray. Those killed were aged 46 to 73, he said. Investigations continue into the circumstances of their deaths.
Investigators were pursuing a possible second gunman after initial interest in two additional suspects, but later said there were no suspects beyond Alexis. The incident has raised questions about security at the Washington navy yard, about 1.6km south of the US Capitol and 4.8km from the White House.
Alexis, a contract employee, had legitimate access to the navy yard and used a valid pass, the FBI said. Authorities did not address how he could have sneaked weapons on to the base. Patricia Ward, who works at the base, said employees do not have to pass through a metal detector when they enter the building.
Police patrol officers and active shooter teams put an end to the rampage, shooting Alexis dead. Washington metropolitan police chief Cathy Lanier said the gun battles produced acts of heroism she could not yet reveal.
It was the worst attack at a US military installation since US Army Major Nidal Hasan opened fire on unarmed soldiers at Fort Hood, Texas, in 2009, killing 13 people and wounding 31 others. Hasan, who said he acted in retaliation for US wars in Muslim countries, was convicted and sentenced to death by a military jury in August.
"We are confronting yet another mass shooting, and today it happened at another military installation, in our nation's capital," said US President Barack Obama, who vowed to enact "sensible" gun control measures after a gunman shot dead 20 school children and six adults at an elementary school in Connecticut in December.
Alexis, a one-time Texas resident who was known to worship at a Buddhist temple, served in the military and most recently was furthering his education while holding a job in the private sector, said his father, Algernon Alexis. "This comes as a complete shock," Alexis said when told his son was the suspected shooter.
Alexis served full time in the US navy reserve from May 2007 to January 2011, becoming an aviation electrician. He received the National Defence Service Medal and the Global War on Terrorism Service Medal, according to a Navy official.
IT consultancy The Experts had hired Alexis recently as a civilian information technology contractor to work on the navy and marine corps intranet and gave him a security clearance classified as "secret", said chief executive Thomas Hoshko. "He did have a secret clearance. And he did have a CAC [common access card]."
Alexis was arrested previously on September 4 2010 in Fort Worth, Texas, on a misdemeanor charge of discharging a firearm. But the case was dropped when investigators determined he was cleaning his gun and it fired accidentally, Tarrant County prosecutors said.
He was also arrested in Seattle, Washington, in 2004 for shooting out a construction worker's car tires in an anger-fuelled "blackout" triggered by perceived "disrespect", according to the Seattle Police Department.
Alexis had developed a love for Thai culture in recent years, learning to speak the language and working at the Happy Bowl restaurant in Fort Worth in 2008, said nearby Thai restaurant owner Tiki Confer (64), adding that he worshipped at a Buddhist temple. "He was a very nice boy. When I saw his picture on the news, I was shocked," she said.
The shooting rattled the US capital, forcing the Federal Aviation Administration to suspend briefly departures at Reagan National Airport. The District of Columbia Public Schools put six schools and an administration building on lockdown as a precaution.
The Washington Nationals baseball team postponed their game against the Atlanta Braves, scheduled for Monday night at nearby Nationals Park.
Lanier said investigators lifted the "shelter in place" for neighbourhoods near the navy yard once they had exhausted all leads for a possible second shooter.
Navy secretary Ray Mabus called the navy yard shootings "an attack on the navy family" and said the shooting revealed a potentially serious security breach.
Personnel are generally banned from carrying weapons on military installations, but most people with proper credentials are not routinely checked for firearms.
Navy commander Tim Jirus, who was in charge of evacuating the building, said he wondered how the suspect gained access. "Right now a lot of people are wondering just how safe the building is or just how safe the office environment is." – Reuters, Sapa