Gunman among 13 killed in Washington shooting rampage

The FBI identifies the alleged shooter as Aaron Alexis, 34, of Fort Worth, Texas

In the deadliest mass shooting in the US since last December's massacre at a Connecticut elementary school, a gunman killed 12 people and wounded 14 others at a US naval base in the heart of the capital before being shot dead.

The Federal Bureau of Investigation (FBI) identified the alleged shooter at the sprawling Washington Navy Yard complex in Southeast Washington not too far from the Capitol and the White House, as Aaron Alexis, 34, of Fort Worth, Texas.

Alexis who received a general discharge in 2011 from the Navy Reserve, was arrested but not charged in a gun incident in Seattle in back 2004 but still had a security clearance with a military contractor that would have allowed him access to the Navy Yard, officials said.

Even as the FBI ruled out any other shooters in the rampage at the headquarters for Naval Sea Systems Command, Metropolitan Police were trying to track down at least one person to determine whether that individual had any involvement.

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

The other possible suspect was described by police as a black male, between 40 and 50, wearing an "olive drab-coloured" military-style uniform.

President Barack Obama called the shooting a "cowardly act" that targeted military and civilians serving their country and vowed that federal and local law enforcement officials would work together to hold accountable those responsible for the shooting.

"They know the dangers of serving abroad," he said, "but today they faced the unimaginable violence that we wouldn't have expected here at home."

The worst shooting incident at a military installation since 13 people were killed at Fort Hood, Texas, in 2009 began unfolding at 8:20 a.m. when several shots were fired inside the southeast Washington facility, where some 3,000 military and civilian employees work.

Police described the alleged suspect as an adult male, about 6 feet tall with a bald head and medium complexion, dressed in a black top and black jeans. He was armed with a semi-automatic AR-15 rifle; another rifle and a semi-automatic Glock handgun.

The injured included a Washington police officer who has been hospitalised, and a base security guard officer, according to police.

Air traffic to Reagan National Airport was briefly suspended after the shooting and ten public and charter schools and a public school administration building in the capital were also placed on lockdown as a precaution.

Officials postponed a Washington Nationals baseball game that had been scheduled for Monday night at Nationals Park, just a few blocks away from the Washington Navy Yard, US Navy's oldest land establishment created in 1799