WASHINGTON — As many as two gunmen launched an attack Monday morning inside the Washington Navy Yard, spraying gunfire on office workers in the cafeteria and in the hallways at the heavily secured military installation in the heart of the nation's capital, authorities said. At least 12 people were killed.

One gunman was dead after he fired on a police officer, and police hunted for a second possible attacker who may have been disguised in a military-style uniform, authorities said. It wasn't clear how the gunman died.

Investigators said they had not established a motive for the shooting rampage, which unfolded less than four miles from the White House. As for whether it may have been a terrorist attack, Mayor Vincent Gray said: "We don't have any reason to think that at this stage."

The FBI took charge of the investigation. The dead gunman was identified as Aaron Alexis by two federal law enforcement officials. One of those officials said Alexis, 34, was from Texas and is believed to have gotten into the Navy Yard by using someone else's identification card. It is not yet clear if that person was an accomplice or if the ID was stolen.

The Navy said in a statement that Alexis was a full-time reservist from 2007 to 2011, who left the Navy on Jan. 31, 2011, as a petty officer 3rd class. It's not immediately clear why he left.

Alexis had been working for the fleet logistics support squadron No. 46, in Fort Worth, Texas. The Navy says his home of record was New York City.

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

In addition to the dead, at least three people were wounded.

The area where the rampage took place, known as Building 197, was part of the headquarters for Naval Sea Systems Command, which buys, builds and maintains ships, submarines and combat systems. The yard is a labyrinth of buildings protected by armed guards at gates and metal detectors, and employees have to show their IDs to come and go.

About 3,000 people work at the headquarters, many of them civilians.

Witnesses described a gunman opening fire from a fourth-floor overlook, aiming down on people in the cafeteria on the main floor. Others said a gunman fired at them in a third-floor hallway. It was not clear whether the witnesses on different floors were describing the same gunman.

Around midday, police said 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. Washington Police Chief Cathy Lanier said it was unclear if the men were members of the military.

But later in the day, police said in a tweet that the man in the tan uniform had been identified and was not involved in the shooting.

It was not immediately clear whether the number of dead included a gunman.

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

A short distance away, security was beefed up at the Capitol and other federal buildings, but officials said there was no known threat.

Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other 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 she also saw the gunman firing 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.'"

Rick Mason, a civilian program-management analyst for the Navy who works on the fourth floor of the building, said a gunman was firing from the overlook in the hallway outside his office.

Shortly after the gunfire, Mason said, someone on an overhead speaker told workers to seek shelter and later to head for the gates at the complex.

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.

Ward said security officers started directing people out of the building with guns drawn.

One person died at George Washington University Hospital of a single gunshot wound to the left temple, said Dr. Babak Sarani, director of trauma and acute care surgery. A police officer and two civilian women were in critical condition at Washington Hospital Center, said Janis Orlowski, the hospital's chief operating officer.

Orlowski said the police officer was in the operating room with gunshot wounds to the legs. The police chief said the officer was wounded when he engaged the shooter who later died.

One woman at the hospital had a gunshot wound to the shoulder. The other had gunshot wounds to the head and hand.

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 about being on lockdown.

"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."

Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. Only security personnel were allowed to be armed on the grounds, but that can include uniformed security officers, civilian contractors and members of the Naval Criminal Investigative Service.

Cmdr. Timothy Jirus said he has a clearance that would allow him to carry a gun on the campus. He has a secure access card that he swipes to get into the headquarters office.

"I think the security is really good, up until today," Jirus said.

Everyone must show an ID to get through a main gate, and at the building entrance, everyone must swipe a badge to pass through either a door or gate, depending on the entrance.

That "makes me think it might have been someone who works here," Mason said.

The Navy Yard has three gates, according to its website. One is open around the clock and must be used by visitors. A second gate is only for military and civilian Defense Department employees. The third gate is for bus traffic.

U.S. Rep. Joe Courtney, D-2nd District, said he was heartbroken over what happened, and shocked since the Washington Navy Yard is such a secure installation.

"Having been in that wing of the Navy Yard, I'm just in disbelief because it's one of the least likely places you would ever imagine that kind of violence and these people who work there are very important to southeastern Connecticut," he said early Monday. "They're our friends and they have been going through a lot with sequestration in the last eight or nine months. I'm just heartbroken about the losses they experienced and I think we should all have them in our thoughts and prayers."

Courtney said he has been in close and frequent contact with the Naval Sea Systems Command for the past several years, since their mission is "tightly connected" to Electric Boat and the Naval Submarine Base in Groton. One of his staff members exchanged e-mails with the NAVSEA commander Monday morning.

NAVSEA engineers, builds, buys and maintains the Navy's ships and submarines and their combat systems.

"They're outstanding people who really care about the highly-skilled workforce we have and about protecting America's industrial base," Courtney said.

The Naval Submarine Base issued a statement in response to the shooting that said the base and its tenet commands "maintain appropriate anti-terrorism and force protection measures consistent with known and perceived threats. As a general rule we do not discuss specifics of the security postures in place."

Day staff writer Jennifer McDermott and Associated Press writers Jesse Holland, Stacy A. Anderson, Brian Witte and Ben Nuckols in Washington contributed to this report.