The Washington massacre gunman used a valid security pass to get inside the navy complex where he killed 12 people but the motive for the shootings is still a mystery, investigators said.

The lone gunman, who was eventually shot dead by police was 34-year-old Aaron Alexis, a defence contract employee and former navy reservist. He was described as a Buddhist who had also had a violent temper, complained about the navy and being a victim of discrimination, and had several run-ins with the law, including two shootings.

The shootings in the Washington Navy Yard reignited the debate over gun control in the United States. President Barack Obama lamented "yet another mass shooting" and promised to make sure "whoever carried out this cowardly act is held responsible." He has been powerless to get gun control legislation passed by Congress.

Yesterday's onslaught at a single building at the highly secure Navy Yard unfolded about 8.20am in the heart of the US capital, less than four miles ) from the White House.

Alexis carried three weapons: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene.

The AR-15 is the same type used in last year's mass shooting at a Newtown, Connecticut, school that killed 20 pupil and six staff. The weapon was also used in the shooting at a Colorado cinema that killed 12 and wounded 70.

For much of the day, authorities said they were looking for a possible second attacker who may have been disguised in an olive-drab military-style uniform. But later they said they were convinced the shooting was the work of a lone gunman, and the lockdown around the area was eased.

"We do now feel comfortable that we have the single and sole person responsible for the loss of life inside the base today," Washington police Chief Cathy Lanier said.
The FBI took charge of the investigation.

The dead ranged in age from 46 to 73. Some were civilian employees and contractors, rather than active-duty military personnel.

Alexis was an employee with The Experts, a company that was a Defence Department subcontractor on a Navy-Marine Corps computer project and had access to the Navy Yard as a defence contractor using a valid pass.

Witnesses described how he opened fire from a fourth-floor vantage point, aiming down on people on the main floor, which includes a glass-walled cafeteria.

In the confusion, police said initially they were searching for two accomplices who may have taken part in the attack - one carrying a handgun and wearing a tan Navy-style uniform and a beret, the other armed with a long gun and wearing an olive-green uniform.

But as the day wore on, they dropped one person and then the other as suspects. As tensions eased, Navy Yard employees were gradually released from the complex, and children were let out of their locked-down schools.