One shooter is dead after opening fire Monday morning at the Washington Navy Yard, and Metropolitan Police Chief Cathy Lanier announced Monday night that authorities believe he was the sole person responsible for the shooting.

Thirteen people were killed, including the shooter, Mayor Vincent Gray announced at a late afternoon press conference. No motive is known at this time.

"It doesn't get much more serious than this," Gray said.

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Multiple news outlets identified Aaron Alexis , 34, as the deceased shooter. Originally of Fort Worth, Texas, Alexis recently began working at the Navy yard as a civilian contractor, NBC reports.

At approximately 2:45 p.m., DC police announced over Twitter that one of two possible additional suspects, a white male in a tan outfit, had been identified and "is not a suspect or person of interest."

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Officials said a suspect began shooting inside the Naval Sea Systems Command Headquarters building at 8:20 a.m . Approximately 3,000 people work in the building.

A "shelter in place" order was issued for Navy Yard personnel. At least six schools were on lockdown as a precaution in the D.C. area, according to CNN .

Speaking with reporters, Navy Commander and witness Tim Jirus said he saw a man get shot in the head right in front of him.

"We were standing in the alley," Jirus said. "[The victim] told me there was a shooter in our building... I heard two more gunshots and he went down while I was talking to him."

Jirus then fled the area. He said he didn't get a look at the shooter.

SWAT crews as well as police from multiple local and federal agencies are on the scene. President Obama has been briefed "several times" about the shooting, Fox News reports.

Monday night, police confirmed the names of 7 people killed in the shooting. At a 10:15 p.m. press conference, Mayor Vince Gray stated that the deceased victims ranged in age from 46 to 73 and all were civilians or contractors, not military personnel.

This is a breaking news story. Check back for more details.