Aaron Alexis has been identified as one of the men behind Monday's shooting at the Washington, D.C., Navy Yard, where 13 at least people died.
Washington Mayor Vincent Gray said that Alexis, a 34-year-old from Fort Worth, Texas, was shot during an exchange with a Metro Police officer.
"As far as we know, it's an isolated incident," Gray said.
"We have no known motive at this stage."
The mayor said there was "no reason at this stage" to believe it was terrorism, but would not rule it out as officials try to investigate what happened.
D.C. Police Chief Cathy Lanier said authorities were seeking a potential second suspect, described as a black male, approximately 40 to 50 years old.
Before a scheduled economic speech, President Barack Obama deplored "yet another mass shooting," this one targeting military and civilian personnel.
“These are men and women who were going to work, doing their job, protecting all of us," Obama said in addressing the Washington Navy Yard shooting.
"They’re patriots, and they know the dangers of serving abroad. But today they faced the unimaginable: violence that they wouldn’t have expected here at home."
"We will do everything in our power to make sure that whoever carried out this cowardly act is held responsible. I want the investigation to be seamless."
Janis Orlowski, chief medical officer at Washington Hospital Center, said one Metropolitan Police officer and two civilians are being treated there.
All three arrived in critical condition, Orlowski said, but are expected to make a full recovery. One was shot in the legs, another in the shoulder. Those victims were in surgery.
Shots were fired at the Naval Sea Systems Command Headquarters building on the base at 8:20 a.m., and a "shelter in place" order was issued for personnel.
Rick Mason, a program management specialist, told Yahoo News he was on the fourth floor when he saw someone with a shotgun aiming down into the atrium.
Approximately 3,000 people work in the building, the Navy said, though it's unclear how many people were inside at the time of the shooting.