WASHINGTON - An employee at a defense contractor used his pass to get into the Washington Navy Yard and went on a deadly shooting rampage Monday, spraying bullets in the hallways and firing from a balcony on workers in an atrium below. Thirteen people were killed, including the gunman.

The motive for the attack - the deadliest mass shooting on a military installation in the U.S. since the tragedy at Fort Hood, Texas, in 2009 - was a mystery, investigators said.

Mayor Vincent Gray said there was no indication it was a terrorist attack, but he added that the possibility had not been ruled out.

The assault at the highly secure Navy Yard unfolded about 8:20 a.m. in the heart of the nation's capital, less than four miles from the White House and two miles from the Capitol.

The gunman, Aaron Alexis, a 34-year-old former Navy reservist whose last known address was in Fort Worth, Texas, died after a running gunbattle inside the building with police, investigators said.

"This is a horrific tragedy," the mayor said.

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 by late Monday night, they said they were convinced the attack was the work of a lone gunman, and the security 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.

President Barack Obama lamented yet another mass shooting in the U.S. that he said took the lives of American "patriots." He promised to make sure "whoever carried out this cowardly act is held responsible."

The FBI took charge of the investigation.

The attack came four years after Army psychiatrist Maj. Nidal Hasan killed 13 people at Fort Hood in what he said was an effort to save the lives of Muslims overseas. He was convicted last month and sentenced to death.

In addition to those killed, eight people were hurt - three of them shot and wounded, according to the mayor. Those three were a police officer and two female civilians, authorities said. They were all expected to survive.

The dead ranged in age from 46 to 73, according to the mayor. A number of the victims were civilian employees and contractors, rather than active-duty military personnel, the police chief said.

At the time of the rampage, Alexis was an information technology employee with The Experts, a company that was a Defense Department subcontractor on a Marine computer project, authorities said.

Valerie Parlave, head of the FBI's field office in Washington, said he had legitimate access to the base as a defense contractor and used a valid pass.

Alexis had been a full-time Navy reservist from 2007 to early 2011, leaving as a petty officer third class, the Navy said. It did not say why he left. He had been an aviation electrician's mate with a unit in Fort Worth.

A convert to Buddhism who grew up in New York City, Alexis had had run-ins with the law over shooting incidents in 2004 and 2010 in Fort Worth and Seattle and was portrayed in police reports as seething with anger.

Witnesses on Monday described a gunman opening fire from a fourth-floor overlook, aiming down on people on the main floor, which includes a glass-walled cafeteria. Others said a gunman fired at them in a third-floor hallway.

Patricia Ward, a logistics-management specialist, said she was in the cafeteria getting breakfast.

"It was three gunshots straight in a row - pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward said.

The FBI would not give any details on the gunman's weaponry, but witnesses said the man they saw had a long gun - which can mean a rifle or a shotgun.

The Washington Navy Yard is a sprawling, 41-acre labyrinth of buildings and streets protected by armed guards and metal detectors, and employees have to show their IDs at doors and gates. More than 18,000 people work there.

The rampage took place at Building 197, the headquarters for Naval Sea Systems Command, which buys, builds and maintains ships and submarines. About 3,000 people work at the headquarters, many of them civilians.

Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and co-workers encountered a gunman in a long hallway on the third floor. The gunman was wearing all blue, he said.

"He just turned and started firing," Brundidge said.

Terrie Durham, an executive assistant with the same agency, said the gunman fired toward her and Brundidge.

"He aimed high and missed," she said. "He said nothing. As soon as I realized he was shooting, we just said, 'Get out of the building.'"

As emergency vehicles and law enforcement officers flooded streets around the complex, a helicopter hovered, nearby schools were locked down and airplanes at nearby Reagan National Airport were grounded so they would not interfere with law-enforcement choppers.

Security was tightened at other federal buildings. Senate officials shut down their side of the Capitol while authorities searched for the potential second attacker. The House remained open.

In the confusion, police said around midday that they were searching for two men 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. Police said it was unclear if the men were members of the military.

But as the day wore on, police 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.

Adm. Jonathan Greenert, chief of naval operations, was at the base at the time the shooting began but was moved unharmed to a nearby military installation.

Anxious relatives and friends of those who work at the complex waited to hear from loved ones.

Tech Sgt. David Reyes, who works at Andrews Air Force Base, said he was waiting to pick up his wife, Dina, who was under lockdown in a building next to where the shooting happened. She sent him a text message.

"They are under lockdown because they just don't know," Reyes said. "They have to check every building in there, and they have to check every room and just, of course, a lot of rooms and a lot of buildings."