Aaron Alexis, 34, of Fort Worth, Texas, died in a gun battle with police during the rampage at Washington Navy Yard.

Eight others were injured in the attack, which began at around 8:20 local time (12:20 GMT) on Monday.

US President Barack Obama ordered flags be lowered to half mast at the White House and on Capitol Hill.

He said he mourned "yet another mass shooting" and condemned the "cowardly act".

No motive is known for the attack, whose dozen victims ranged in age from 46 to 73, said the authorities.

Police initially responded to reports of two other armed suspects, but after exhaustive searches Washington Police Chief Cathy Lanier said on Monday night they were all but certain that Alexis was the only gunman.

Witnesses said the suspect 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.

Chief Lanier told reporters that the attacker "was engaged in shooting with police officers" when he died. "There is no question he would have kept shooting," she added.

Valerie Parlave of the FBI said that the suspect, who was employed by a subcontractor that was working on a US Navy intranet network, had a valid pass to enter Washington Navy Yard.

Alexis was formerly a petty officer 3rd class who served full-time in the naval reserve from 2007-11, said the US Navy.

It was not immediately clear why he left the military, but unnamed military officers told US media there had been a series of misconduct issues during his service.

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".

And he 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 addition to those killed in Monday's rampage, one police officer was shot in the legs, a female civilian was shot in the head and hand, and another woman was treated for a shoulder wound.

All three were expected to recover, said Washington Hospital Center officials.

Five others required medical treatment, mostly for minor injuries, said Washington DC Mayor Vincent Gray.

As news of the shooting emerged, emergency vehicles and police surrounded the complex, while helicopters hovered overhead.

The US Senate buildings and six nearby schools were temporarily placed on lockdown as a precaution, while flight departures were briefly suspended at Reagan National Airport.

Patricia Ward, who works at Washington Navy Yard, said she had heard three shots, a pause, and then four more.

"Everybody was panicking and trying to decide which way to get out," she told reporters.

The Washington Nationals baseball team called off their game against the Atlanta Braves on Monday night at Nationals Park, not far from Navy Yard.

Monday's mass shooting occurred at a building that serves as the headquarters for the Naval Sea Systems Command, which builds and maintains ships and submarines for the Navy.

Washington Navy Yard is the US Navy's oldest shore installation, first opened in the early 19th Century, according to the Navy.

They said that the unprecedented salvage effort "reached degree zero [vertical], which was our target".

In the operation that took all of Monday and most of the night, they used cables and metal boxes filled with water to roll the ship onto a platform.

The Costa Concordia capsized in January 2012, killing 32 people.

The bodies of two of the victims of the disaster, by the island of Giglio, have never been found. There are hopes that they may be located during the operation.

The ship was declared completely upright shortly after 04:00 local time (02:00 GMT) on Tuesday.

Franco Gabrielli, the head of Italy's Civil Protection Authority, said the vessel was now sitting on a platform built on the sea bed.

"A perfect operation, I must say," said Franco Porcellacchia, leader of the technical team for Costa Cruise, the owner of the ship.

He added that no environmental spill was detected so far.

"I think the whole team is proud of what they achieved because a lot of people didn't think it could be done," said salvage master Nick Sloane - but he added "it's not over yet".

When the vessel was finally righted in the early hours of Tuesday morning, there was a giant cheer from people gathered at Giglio harbour, says the BBC's Matthew Price, and rescue workers have been out celebrating with coffees.

As daylight broke, the now-upright, brown hulk of the ship was visible - its hull muddy and crushed from 20 months spent submerged on its side.

The man, Ji Zhongxing, said he had been paralysed after being beaten by security agents in China in 2005.

No one was killed in the blast, although Mr Ji suffered injuries and was taken to hospital.

If found guilty, he could face three to 10 years in jail for endangering public safety, state media report.

Photos in state media showed Mr Ji giving evidence from a hospital bed to the court.

According to the verified microblog feed of the Beijing court, Mr Ji said he did not deliberately detonate the home-made device and that he regretted his actions.

His lawyer, Liu Xiaoyuan, said that Mr Ji "took out the explosives at the airport to raise awareness about his plight", and "detonated the explosives by mistake" when he was putting them down.

"This is a case of an explosion caused by negligence," he wrote on his microblog.

Prosecutors said that Mr Ji had endangered public safety and should be held criminally responsible for the explosion, news agency China News Service reported.

The 34-year-old from central Shandong province had been petitioning the authorities over his lower limb paralysis, which he said was caused by a beating from several security officers in Dongguan, south China's Guangdong Province, in 2005.

Mr Ji was also apparently dissatisfied with the way his complaints against the authorities had been dealt with.

His case drew widespread sympathy from the Chinese public, the BBC's Martin Patience in Beijing reports.

While an extreme example, it exposed the immense frustrations many feel in China when trying to seek justice, our correspondent adds.

There were reports of petitioners gathering outside the court, expressing support for Mr Ji and asking for their grievances to be considered.

There was also considerable sympathy for Mr Ji on China's microblogs.

Sina Weibo user nymfj wrote: "I hope he is pardoned", while user Let Go of Your Arrogance asked: "Has anyone investigated the case where Ji Zhongxing was beaten?"

Sina Weibo user Old Yip's Comments wrote: "Ji Zhongxing is a man pushed to the brink by Chinese society. From media reports, you can see that he did not want to harm anyone with his 'explosion', he only wanted to gain attention for his plight. I recommend he is given a reprieve. At the same time, the Guangdong government must give him compensation, and support him for the rest of his life."

However, some users argued that Mr Ji should be punished.

"These cases show that terrorism must be opposed everywhere... it must all be dealt with in accordance to the law," user Zhang Yiwu wrote.

According to China Central Television, Dumex allegedly paid doctors and nurses in the northern city of Tianjin to promote its baby formula products.

The report, which cited an unidentified former sales manager, comes amid an anti-corruption drive by Beijing.

Dumex said it was "extremely shocked" by the allegations.

"Dumex Baby Food Co strictly adheres to Chinese laws and regulations," a company spokeswoman said in a statement.

The latest allegations against Danone come amid a government-led crackdown on corruption in its $350bn (£220bn) healthcare market.

Some inquiries have targeted foreign firms, including British pharmaceutical giant GlaxoSmithKline, which is being investigated for allegedly using inducements to promote the sale of its medicines.

In the CCTV report, Danone's Dumex brand is accused of giving "sponsorship fees" or payments of up to 10,000 yuan ($1,632) to hospital staff.

The French food giant has faced numerous problems in China this year.

Danone cut prices for its infant milk formula products by as much as 20% after China's top economic planning body fined it in August for price-fixing.

The firm also had to issue a precautionary recall of its milk formula products last month after one of its suppliers, Fonterra, said some items may have been contaminated.

China is an important market for companies such as Danone, which is the country's third-largest baby formula provider.

Danone's baby nutrition unit also makes more than 20% of its overall sales from China.

Demand for foreign brands has surged in China, after tainted milk scandals in recent years led to a distrust of local producers.

According to some estimates, foreign brands now account for about half of all infant milk sales in China.