Washington's chief of police says law enforcement officials now believe that Aaron Alexis was the "single and sole person" who opened fire in the U.S. capital's navy yard Monday, killing 12 people at the largest of the U.S. navy's five system commands.

D.C. Police Chief Cathy Lanier announced that a "shelter-in-place" order for a neighbourhood around the ship yard has been lifted, as investigators have "exhausted all means available" to find a possible second gunman.

"We do feel comfortable right now that we have the single and sole person responsible for the loss of life," Lanier said in a late-night news briefing .

Alexis, a 34-year-old Texas man and former navy reservist, was shot and killed after a running gunbattle inside the building with police, investigators said.

Police had earlier said there was one other potential perpetrator still at large, an armed man dressed in military garb who was being sought by police.

A third man, also clad in military-style outfit, was cleared as a suspect, police said.

Another 14 people were injured, some critically, including a local police officer. The names of the victims have not yet been released, but their ages range between 46 and 73.

U.S. navy Secretary Ray Mabus called the shooting "an attack on our navy family" in a brief statement released on YouTube, and offered the navy's support to the victims and their loved ones.

"We will come together as a family to deal with this," he said.

Meanwhile, questions were arising about a possible motive and about the gunman, Alexis.

The FBI appealed to the public to come forward with information about Alexis, born in the borough of Queens in New York City.

Alexis received a general discharge from the reserve in 2011 after a series of misconduct issues, according to a navy official who spoke on condition on anonymity. The official declined to provide details on the types of misconduct in Alexis's record.

A brief, official bio released by the navy indicated Alexis served as an electrician and was last stationed in Fort Worth, Texas.

More recently Alexis was an employee of a company called "The Experts," a subcontractor for an HP Enterprise Services military contract, Hewlett-Packard said.

HP said the contract was "to refresh equipment used on the Navy Marine Corps Intranet (NMCI) network."

He had "secret" clearance and was assigned to start working at the navy yard as a civilian contractor with a military-issued ID card.

The city's mayor, Vincent Gray, said officials had no reason to believe the shooting was an act of terrorism.

"We'll continue to seek information about what the motive is," he said. "We don't have any reason at this stage to suspect terrorism, but certainly it has not been ruled out."

In remarks from the White House, a terse U.S. President Barack Obama pointed out the regularity with which such shootings afflict the nation. The president tried unsuccessfully to push tougher gun control laws through Congress earlier this year following the horrific shooting in small-town Connecticut in December.

"We are confronting yet another mass shooting, and today it happened at a military installation in our nation's capital," Obama said. "We will do everything in our power to make sure whoever carried out this cowardly act is held responsible."

Panic and terror

Initial reports suggested the dead shooter entered headquarters at the navy yard complex with stolen ID, fatally shot a security guard and was targeting specific people during his subsequent shooting rampage.

Eyewitnesses said the gunman situated himself at a fourth-floor overlook and was shooting down at people at the cafeteria on the first floor, creating panic and terror as people tried to flee the area. He had an AR-15 assault rifle, a shotgun and a pistol, police added.

"I heard three shots — pow, pow, pow. Thirty seconds later I heard four more shots," said Patricia Ward, a logistics management specialist who was in the cafeteria at the time.

"A lot of people were just panicking. There were no screams or anything because we were in shock."

It emerged Alexis was the subject of a police investigation in Fort Worth three years ago when he was arrested for discharging a firearm and shooting a hole through the ceiling of his home and into a neighbour's apartment.

There was no indication he was ever charged, and Alexis told police he was cleaning his gun went it went off accidentally.

But in the police report, the neighbour said she suspected Alexis had deliberately fired the gun through her floor because he had complained she made too much noise. She added she was "terrified" of Alexis.

He was also investigated for shooting out the tires of another man's car in Seattle in 2004. His father told police that his son suffered from post-traumatic stress disorder and had helped in rescue efforts in New York City following the terrorist attacks of Sept. 11, 2001.

Airport locked down

Previous Next Monday's shootings prompted the temporary lockdown of D.C.'s Ronald Reagan National Airport and schools surrounding the navy yard, a historic complex in the city's southeast that dates back to the 18th century and is now the workplace for about 3,000 people.

A major-league baseball game between the Washington Nationals and the Atlanta Braves was also postponed. The baseball stadium is just a couple of blocks from the scene of the shootings.

Well into the evening, as navy yard employees were still filing out of the building, the city was on edge. Reports of shots fired at the White House turned out to be someone tossing firecrackers over the presidential residence's iron fence, the Secret Service said.

Police quickly apprehended the man.

The shooting made headlines on news sites around the world. A Russian official even weighed in — when the confirmed death toll was just seven — to mock Obama's insistence in a speech last week on Syria that the U.S. was "exceptional."

"A new shootout at Navy headquarters in Washington — a lone gunman and 7 corpses. Nobody's even surprised anymore," tweeted Alexey Pushkov, a member of the Russian parliamentary foreign affairs committee.

"A clear confirmation of American exceptionalism."

Sen. Dianne Feinstein, one of the U.S. Senate's biggest gun control advocates, called once again for stricter gun laws in the aftermath of yet another mass shooting.

"This is one more event to add to the litany of massacres that occur when a deranged person or grievance killer is able to obtain multiple weapons -- including a military-style assault rifle – and kill many people in a short amount of time," the California Democrat said in a statement.

"When will enough be enough? Congress must stop shirking its responsibility and resume a thoughtful debate on gun violence in this country. We must do more to stop this endless loss of life."