At least 13 dead in Washington Navy Yard shooting...

At least 13 people are dead and several others were wounded after a gunman opened fire at the Washington Navy Yard on Monday, police said, spreading fear and chaos across the region as authorities sought to contain the panic. The incident, in which the death toll rose almost hourly, represents the single worst loss of life in the District since an airliner plunged into the Potomac River in 1982, killing 78.

D.C. Police Chief Cathy L. Lanier and Mayor Vincent C. Gray announced the mounting number of casualties in a series of news conferences. The suspected shooter, identified by the FBI as Aaron Alexis, 34, living in Fort Worth, is among the 13 dead. Alexis was a military contractor, one official said.

But even hours after the rampage began, it was still unclear whether the shooting was the act of a lone gunman, or if other shooters were involved. Lanier initially said authorities were looking for two more potential shooters dressed in military style clothing. But shortly after she announced a detailed description of two suspects, city officials said one had been located and cleared.

Gray said no motive is known yet. He said there is no reason to believe it was an act of terrorism, though he added that he could not rule it out.

Gray said that in addition to those killed, about a dozen people were hurt. It was not clear whether those people were shot or suffered other injuries, he said.

Lanier described the other possible suspect, who has not been located, as a black man in his 40s with gray sideburns, wearing an olive-drab military-style uniform. He, and the man who was cleared, came under suspicion when they were seen on surveillance videos.

Police are asking anyone with information on the suspect to call 202-727-9099.

Alexis was armed with an assault rifle and a handgun, two law enforcement officials said. One said he also had a shotgun. One official said all the weapons have not been accounted for.

The first, sketchy details about the suspect offered few hints about what may have gone wrong.  

Alexis grew up in Brooklyn with his mother, Sarah, and father, Anthony Alexis, according to his aunt Helen Weeks.

“We haven’t seen him for years,” Weeks said of her nephew in a telephone interview. “I know he was in the military. He served abroad. I think he was doing some kind of computer work.”

Alexis spent nearly four years in the Navy as a full-time reservist from May 2007 until he was discharged in January 2011, according to a summary of his personnel records released by Navy officials at the Pentagon.

The officials said they were still researching whether Alexis had been employed as a defense contractor or a civilian employee of the Navy, and were uncertain if he was assigned to work at the Navy Yard.

He achieved his final rank of Aviation Electrician’s Mate 3rd Class in December 2009. Officials said they did not immediately know the reasons for his discharge.