By MICHAEL D. SHEAR, EMMARIE HUETTEMAN, ABBY GOODNOUGH and MICHAEL S. SCHMIDT From New York Times

WASHINGTON — At least 13 people, including one gunman, were killed, and the police were looking for other potential suspects, in a shooting Monday morning at a naval office building not far from Capitol Hill and the White House, police officials said.
One police officer was in surgery after being shot in an exchange of fire with a gunman, said Chief Cathy L. Lanier of the Metropolitan Police Department. The shootings took place at the Washington Navy Yard, in the southeast part of the city.

Senior law enforcement officials identified the gunman as Aaron Alexis, 34. He was identified through his fingerprints. As of Monday night, investigators were operating on the belief that Mr. Alexis acted alone, despite earlier statements from Washington law enforcement officials that there were two other gunmen.

According to the Navy, Mr. Alexis enlisted as a full-time reservist in May 2007 and left the service in January 2011. He served as an aviation electrician, and the highest rank he achieved was mate third class. From February 2008 to January 2011, he was assigned to Fleet Logistics Support Squadron 46, in Fort Worth.

The Navy said that Mr. Alexis had been awarded the National Defense Service Medal and the Global War on Terrorism Service Medal.

Shortly after 4 p.m., the F.B.I. released a “Seeking Information” bulletin asking for the public’s help in learning more about Mr. Alexis. The bulletin, which had two photographs of Mr. Alexis, said that he was born in Queens and was 6 feet 1 inches tall and weighed 190 pounds.

Valerie Parlave, the assistant director of the Washington field office of the F.B.I., urged members of the public to look at pictures of Mr. Alexis on the F.B.I. Web site and to call with any information they may have about him.

“No piece of information is too small,” she said. “We are looking to learn everything we can about his recent movements, his contacts and his associates. We ask the public to look at the photos of the deceased shooter.”

Three weapons were found on the gunman: an AR-15 assault rifle, a shotgun and a semiautomatic pistol, an official said.

“It’s hard to carry that many guns, so there is some thinking that he may have taken some of them from security or whoever else he shot,” the official said.

One federal law enforcement official said the suspected gunman had family in New York, a mother and a sister or sisters, but had not lived there.

The official indicated that the gunman was captured on video as he entered the building before the shooting and said that it was possible that other portions of the episode were also recorded by cameras inside the building.

“We’re continuing to see if there are in fact additional shooters, but we have nothing to indicate that yet,” the official said.

As the day wore on, officials released conflicting information — that they later retracted — about the search for two possible gunmen, one wearing a khaki Navy uniform and carrying a handgun, the other who was believed to be carrying a “long gun.”

One law enforcement official involved in the investigation said that the authorities had received few reports from witnesses of seeing gunmen who fit the descriptions that the city officials had provided. But, he added, “There would have been a whole lot more damage if there had been three gunmen.”

Officials in Washington did not move to lock down the city after the shootings. The Metro, the city’s subway system, continued to operate normally. Out of an “abundance of caution,” Terrance W. Gainer, the Senate sergeant-at-arms, put the Senate complex into lockdown at just after 3 p.m. The Senate had recessed in the early afternoon.

Police officials said that at least two police officers were shot, and that three people were in critical condition.

Navy Secretary Ray Mabus said Monday afternoon that the “Navy family suffered a horrific loss today” as he visited three of the victims at a hospital.

Doctors at MedStar Washington Hospital Center said that the three victims from the shooting were all expected to recover. Two of the three were in surgeries that would last hours, but all three were stable, hospital officials said.

One police officer was in surgery after sustaining multiple gunshot wounds to his legs. A woman who was shot in the shoulder was also in surgery. A third victim, a woman who was shot in the head and hand, did not require surgery because the bullet did not penetrate her skull.

The shooting started at 8:20 a.m., the Navy said on Twitter. Workers were told to shelter in place, and emergency personnel were on the scene.

Hundreds of police officers and naval officers surrounded the Naval Sea Systems Command headquarters, where about 3,000 service members, civilians and contractors work on the Navy’s fleet. Security at the Capitol was enhanced, and the local news media reported that several schools were put on lockdown. Security around the Pentagon was visibly increased after the shooting.

Investigators were trying to determine whether any of the suspected gunmen had official access to the Navy Yard. The site is protected by a high wall, with entry through checkpoints that require official identification. However, under the “force protection status” that was believed to have been in effect early Monday, someone with official access to the site could have driven a car into the parking lot without having the trunk inspected, or could have entered on foot without a bag being searched.

President Obama said that “whoever carried out this cowardly act” would be held responsible, and he praised as “patriots” the victims of the shooting.

“We still don’t know all the facts, but we do know that several people have been shot and some have been killed,” Mr. Obama said before beginning an event marking the fifth year since the 2008 economic collapse. “So we are confronting another mass shooting, and today it happened on a military installation in our nation’s capital.”

Mr. Obama said that he had told officials to make sure the investigation was “seamless,” and that there would be time to honor the victims as more became known about who they were.

“They are patriots,” Mr. Obama said. “They know the dangers of serving abroad. But today they faced the unimaginable violence that they wouldn’t expect here at home.”

The Navy Yard sits on the waterfront along the Anacostia River in southeast Washington, near Nationals Park, where Washington’s baseball team plays. It is also at one end of the 11th Street Bridge, one of the major thoroughfares that bring traffic into the city from Maryland.

Shortly after 3 p.m., the Washington Nationals postponed their game against the division-leading Atlanta Braves, which had been scheduled for 7 p.m. The Nationals’ Web site said “Postponed: Tragedy,” and said the teams would play a doubleheader on Tuesday instead.

Adm. Jonathan W. Greenert, a four-star admiral, a member of the Joint Chiefs of Staff and the chief of Naval Operations, was evacuated from his residence at the Navy Yard complex, along with his wife, shortly after the first report of shots fired, according to Navy officials.

Military helicopters circled the facility as police vehicles and other emergency vehicles rushed to the Navy Yard. A helicopter lowered a basket to the roof of one of the buildings and appeared to be taking away victims.

Officials at the nearby Reagan National Airport briefly stopped airplanes from taking off or landing because of the shooting, according to a spokeswoman at the airport’s media office. The stoppage began just before 10 a.m. and was lifted at 10:14 a.m., said Laurie Weaver, the spokeswoman.

The Senate majority leader Harry Reid, Democrat of Nevada, in the afternoon adjourned the Senate for the day.

“My sympathies are, of course, with those families who died, with those that have been injured, and my wishes go out to all those who work in the Navy Yard complex, which is just a short distance from here,” Mr. Reid said on the Senate floor. “And my thanks go out to the brave law enforcement officials who were on the scene and who put their lives on the line and today we realize that they really do put their lives on the line to keep this Capitol complex safe and the city safe.”

He concluded: “In light of the events at the Navy Yard, we’ve decided to recess the Senate until tomorrow morning.”

The Senate recessed just after 2 p.m., when Monday’s session was expected to start, and it will pick back up Tuesday at 10 a.m.

Senator Mitch McConnell, Republican of Kentucky and the Senate minority leader, also spoke briefly to express his condolences and thank the first-responders.

“We’re thinking about today’s tragic shootings at the Navy Yard, about the victims and about their families,” Mr. McConnell said. “The men and women in our military courageously put their lives on the line in dangerous places around the world. It’s painful to think about them having to worry about their safety when they come home, too.”

Patricia Ward, a logistics management specialist from Woodbridge, Va., was in the cafeteria on the first floor when the shooting started. “I heard three gunshots, pow, pow, pow, straight in a row,” she said. “About three seconds later, there were four more gunshots, and all of the people in the cafeteria were panicking, trying to figure out which way we were going to run out.”

“The workers in the cafeteria wanted us to stay there,” she added, “but I just ran, I ran out the side doors.”

She added that someone had pulled a fire alarm, and that many people just started to run away.

Employees who had been evacuated from the building described a chaotic situation as an individual armed with a rifle roamed the hallways shooting at people.

Cmdr. Tim Jirus said he was on the fourth floor of the building when he heard gunshots and saw people start running through the office.

“I heard a number of what I thought were like cap-gun shots, based on distance, inside the building,” he said.

Commander Jirus said he was at the back of the building working to get people out when a man came out of a maintenance building and approached him, asking about the shooting. Moments later, the man, a civilian, was shot in the head, Commander Jirus said.

“We had a conversation for about a minute,” he said. “I heard two gunshots, and he went down, and then I ran back here.”

Holding a radio as he waited outside the Navy Yard Metro station, Commander Jirus said he had heard that another man in his office, also a civilian, had been shot and evacuated to a hospital.

Asked how he escaped when the man next to him was shot, he said: “Luck. Grace of God. Whatever you want to call it.”

William K. Rashbaum contributed reporting from New York.

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