The suspect was identified by the FBI as Aaron Alexis of Fort Worth, Texas. Washington D.C. police chief Cathy Lanier told reporters that Alexis “was engaged in shooting with police officers” when he died.

“We have no indication of motive at this time,” Lanier said.

Valerie Parlave, assistant director in charge of the Washington field office of the FBI, asked the public for help with information about Alexis. Authorities said they were searching for another possible gunman wearing military-style clothing.

Earlier, officials said they were looking for two men, but then said police had established one of them was not a suspect in the shooting, which began at about 8.30 a.m. EDT (1230 GMT).

Washington D.C. Mayor Vincent Gray said 13 people were killed and a dozen or more injured at the Naval Sea Systems Command, one of five such commands where civilians, military personnel and contractors build and maintain Navy ships and submarines. About 3,000 people work there.

Patricia Ward, who works at the Navy Yard, said she heard three shots “pow, pow, pow” and then four more shots after a pause.

“Everybody was panicking and trying to decide which way to get out,” Ward told reporters.

Security guards told people to “run, run, run,” Ward said.

“You just want to know why,” said David Reyes, a tech sergeant stationed at Andrews Air Force Base who rushed to the Navy Yard when he received a text message from his wife, who works there.

Reached by telephone, Aaron Alexis’ father, Algernon Alexis, seemed stunned by the news his son may have been involved.

“This comes as a complete shock,” he said and then asked a Reuters reporter how he could reach authorities leading the investigation.

He said his son was former military and now studying while working in a computer-related job for a private company.

ANOTHER MASS SHOOTING

President Barack Obama was briefed on the shooting and then talked about it at the start of a speech on the U.S. economy.

“We are confronting yet another mass shooting, and today it happened at another military installation, in our nation’s capital,” said Obama, who vowed to enact “sensible” gun control measures after a gunman shot dead 20 school children and six adults at an elementary school in Connecticut last December.

“They know the dangers of serving abroad, but today they face the unimaginable violence that they wouldn’t have expected here at home,” Obama said.

For a graphic of the area of the shooting, please click on http://link.reuters.com/dym23v

The Washington shooting happened less than three weeks after U.S. Army psychiatrist Major Nidal Hasan was sentenced to death for murdering 13 people in 2009 at Fort Hood, Texas, where he gunned down unarmed soldiers in what he later called retaliation for U.S. wars in Muslim countries.

Among the injured in Monday’s shooting was a District of Columbia police officer and one other law enforcement officer, officials said. Washington Hospital Center said it treated three gunshot victims, who were in critical condition.

At George Washington University, a man in his 60s died of a single gunshot wound to his head, said Babak Sarani, head of trauma surgery.

The man was shot in his left temple and the injury “was not survivable by any stretch,” Sarani told reporters, adding he was unsure what type of weapon was used or whether the bullet had exited the victim’s body.

SECURITY BREACHED

The shooting revealed a potentially serious security breach.

The command where the shooting takes place requires two separate identification badges, one to get on the base and another to access the building, according to a source who works at the Navy Yard and requested anonymity. Military personnel are generally banned from carrying weapons on military installations but most people with proper credentials are not routinely checked for firearms.

“It will be interesting to see as this develops who the shooter is, how he got in,” said Navy Commander Tim Jirus, who was in charge of evacuating the Sea Command building. “Right now a lot of people are wondering just how safe the building is or just how safe the office environment is.”

Dozens of police and emergency vehicles surrounded the complex in southeast Washington, which is about a mile (1.6 km) south of the U.S. Capitol and 3 miles (5 km) from the White House. Helicopters circled the headquarters with some touching down on the building’s roof. DM