WASHINGTON — Law enforcement officials said the death toll in the shooting at the Washington Navy Yard has risen to 13, including the shooter, whom officials identified as 34-year-old Aaron Alexis of Fort Worth, Texas.

Three other people were being treated at a local hospital and were expected to fully recover, hospital officials said.

District of Columbia police say they are no longer looking for other gunmen in the mass shooting at the Washington Navy Yard on Monday morning, indicating that the victims of the attack were all killed by a single shooter.

Federal officials have identified Alexis, a government civilian contractor who was new to the Washington, D.C., area, according to a law enforcement official who spoke on the condition of anonymity to discuss the investigation. He may have used the identification badge of another person to gain access to the base.

Federal officials say Alexis was arrested in Fort Worth on Sept. 5, 2010, on suspicion of discharging a weapon. The Tarrant County district attorney did not prosecute.

The three victims, including a police officer, were brought to MedStar Washington Hospital Center.

In a news conference, Dr. Janis Orlowski, the center’s chief medical officer, said the chances of their survival were good. She confirmed that all three victims were alert and speaking.

The police officer was shot in both of his legs. The other two victims are both female civilians. One suffered injuries to her shoulder, the other to both her head and hand. All three are confirmed to have been wounded inside the Navy Yard.

The neighborhood near the naval facility remained sealed off, with residents locked out and nearby schools locked down six hours after the first reports of the violence came in at 8:20 a.m. The Senate was placed on a preventive lockdown early Monday afternoon, with votes postponed. The lockdown was later partly lifted.

“We have no indication of any motive at this time,” said Cathy Lanier, chief of the Metropolitan Police Department, at her second news conference of the day, “there are very few questions we can answer at this point.”

The FBI was taking the reins of the investigation, the police chief said. Lanier credited D.C. police and the U.S. Park Police for preventing even more bloodshed in the morning rampage.

“I think the actions by the police officers, without question, helped to reduce the numbers of lives lost,” Lanier said, calling the actions of first responders “nothing short of heroic.”

Because the attack happened at a military facility in the capital, there were immediate fears that terrorism might be involved. D.C. Mayor Vincent Gray told reporters there was no evidence one way or another on terrorism.

At an earlier news conference, Lanier said two men, one white and one black, between 40 and 50, were sought for questioning. The white male was said to have been dressed in a tan military-style uniform with a beret-like hat, the black male in an olive-colored military-style uniform. Later in the afternoon, police confirmed that the man in the tan outfit had contacted authorities and was cleared.

The other suspect Lanier said witnesses had described, a black male in military-style clothes carrying a rifle, is believed to have been Alexis, who was killed by law enforcement officers in a firefight about 10:15 a.m. EDT, roughly two hours after the shooting began.

The area around the Navy Yard, not far from Capitol Hill, had been sealed off by layers of law enforcement personnel from local and federal agencies. Senate sergeant-at-arms Terrance Gainer issued a statement midafternoon announcing that no one would be allowed in or out of Senate offices.

“In light of the uncertainty surrounding the shooting at the Navy Yard this morning and particularly the possibility of suspects remaining at large, we have decided to lock down the Senate complex,” the statement said. “This will be in effect until we deem the situation safe in the neighboring community. We do not have any information to suggest the Senate, its members or staff are in any danger, but out of an abundance of caution, we feel this is the best course of action to keep everyone safe.”

The shooting began inside the Naval Sea Systems Command headquarters building, a workplace for 3,000 people, according to a press release from Naval District Washington.

Contractor Sean Carroll described a chaotic scene on the second floor once the shooting started near a cafeteria atrium in the building.

“People didn’t realize what you were supposed to do,” Carroll said, “just heard the sounds. It was really loud. You could hear the gunshots. That’s a surreal thing. You’re not really thinking. But it wasn’t like, ‘Hey, what’s going on?’ You know, with the world we live in.”