By Chelsea J. Carter and Brian Todd
WASHINGTON (CNN) -- There are more questions than answers about who was behind Monday's rampage at the Washington Navy Yard even as authorities confirmed the identity of a dead gunman and searched for another possible suspect.
Authorities say at least 13 people were killed in a rampage at the headquarters for Naval Sea Systems Command in southeast Washington, where witnesses described a terrifying, chaotic scene that began unfolding at 8:15 a.m. ET.
Authorities have identified the dead gunman as 34-year-old Aaron Alexis, a former Navy reservist and a current military contractor, the Washington FBI Field Office told CNN. His identity was confirmed by fingerprints and a picture ID card, the FBI said.
"We have no known motive at this stage. There were people who asked earlier if this was a terrorist event. We don't have any reason to think that at this stage. But we are continuing that investigation to try to determine what the motive is," Mayor Vincent C. Gray told reporters.
"We also don't know for certain if there are other shooters," he added.
Police also have cleared one of at least two people they were looking for in connection with the shooting. That man, a white male wearing a Navy uniform, "has been identified and is not a suspect or person of interest," Metropolitan Police said in a post on Twitter.
Authorities are, however, looking for a black male, between 40 and 50, with a medium complexion and gray sideburns who was reportedly wearing an "olive drab-colored" military-style uniform, Cathy Lanier, the chief of the Metropolitan Police Department, told reporters.
He was believed to be carrying what Lanier described, as a "long gun."
Alexis was carrying a military-contractor ID that matched his appearance, a Metropolitan D.C. police official told CNN on condition of anonymity.
That may explain how Alexis was able to get into the building, which requires military-approved identification to gain access.
Alexis, who was from New York City, served as a full-time Navy reservist between 2007 and 2011, according to military records.
In the Navy, Alexis achieved the rank of aviation electrician's mate 3rd class, working on aircraft electrical systems, the records show.
It was not immediately clear what role Alexis held as a military contractor.
His last known address was in Fort Worth, Texas, though it is not clear if he was still living there at the time of the shooting.
While the police chief would not say why authorities believe there may be others involved, witnesses have given CNN and other news organizations several descriptions that indicate there may have been more than one shooter.
President Barack Obama, meanwhile, said the shooting that erupted inside Building 197 "targeted military and civilian personnel."
"We will do everything in our power to make sure whoever carried out this cowardly act is held responsible," Obama said. "We still don't have all the facts."
Two people told CNN affiliate WJLA that they heard a fire alarm go off in the building where they worked and then saw a tall African-American man with a rifle down the hallway as they exited the building.
"He aimed the gun and fired our way," a man who identified himself as Todd Brundidge told WJLA, adding, "I couldn't believe it."
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