Aaron Alexis, 34, of Fort Worth, Texas was killed following a shoot-out with D.C. and U.S. Park police.
Law enforcement from several agencies including the F.B.I., D.C. police, U.S. Capital police and military police responded to reports of multiple shots fired around 8:20 a.m.
Witnesses described a gunman opening fire from a fourth-floor overlook, aiming down on people in the first-floor cafeteria. Others said a gunman fired at them in a third-floor hallway.
Patricia Ward, a logistics-management specialist, said she was in the cafeteria getting breakfast.
"It was three gunshots straight in a row - pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward told reporters several blocks away from the Navy Yard.
Ward said security officers started directing people out of the building with guns drawn.
U.S. Park police helicopters dipped down to the roof of a Navy building and lifted people out in baskets.
A shelter-in-place was immediately ordered at the Naval Sea Systems Command's headquarters, where about 3,000 people work.
The 34-year-old, born in Queens, N.Y., was said to be carrying an AR-15, double barrel shotgun and third handgun during Monday's rampage.
No indication of a possible motive is known at this time. Authorities say they will examine all possible motives, including terrorism.
"We don't know what the motive is at this stage. Obviously, we will continue to seek information about what the motive is, but we don't have any reason at this stage to suspect terrorism, but certainly it has not been ruled out," said D.C. Mayor Vincent Gray.
In addition to those killed at the Navy Yard, eight people were hurt, including three who were shot and wounded, according to the mayor. Those three were a police officer and two female civilians, authorities said. They were all expected to survive.
Officials said the victims who died ranged in age from 46 to 73 years old. No victims were active duty military personnel; all are described as civilians and contractors:
- Michael Arnold, 59
- Sylvia Fraiser, 53
- Kathy Gaarde, 62
- John Roger Johnson, 73
- Frank Kohler, 50
- Kenneth Bernard Proctor, 46
- Vishnu Pandit, 61
The names of the rest are being withheld pending next of kin notification.
Shooting suspect was in Navy reserves
Police said the gunman used a valid pass to get onto the base before launching the attack. Sources tell ABC News that Alexis' rental car was found on the naval base. So it's looking increasingly likely he drove legally onto the base with a semi-automatic rifle inside his own vehicle.
Alexis worked for Hewlett-Packard as an IT subcontractor for the Navy, the company said. He was an employee of a company called "The Experts" that refreshed equipment used on the Navy Marine Corps Intranet network. It was unclear if he was working at the Navy Yard itself.
"We are deeply saddened by today's tragic events at the Washington Navy Yard," an HP spokesperson said in a statement. "HP is cooperating fully with law enforcement as requested."
Alexis was a former full-time Navy reservist from 2007 to 2011. He was detached from service in January of 2011 for a series of misconduct issues, according to a Navy official. No other information was released.
Alexis reached the rank of Aviation Electrician's Mate 3rd Class. He left with a general, not an honorable, discharge.
"If he got out under good terms, it should have said honorable discharge, and yet it said general discharge," said retired Marine Lt. Col. Hal Kempfer. "What it tells me is he had a rather checkered career in the Navy, if you will. Something went wrong."
Kempfer says that past wouldn't necessarily stop him from working at the Navy Yard recently as a civilian contractor.
"Once you have a military I.D. card and work at the base, there is an assumption you're just supposed to be there," said Kempfer.
Alexis was most recently stationed at Naval Air Station Fort Worth where he worked for the fleet logistics support squadron No. 46, according to Lt. Cmdr. Sarah Blansett, a Navy spokesman.
Alexis' friends and family said they are shocked. They say he was not a violent, loner type.
"He seemed like the kind of guy that liked to have weapons, but he never acted like he would hurt anyone," said his friend, Michael Rotrovato.
Shooting suspect was arrested in the past
According to Tarrant County, Texas officials, the deceased gunman was arrested on Sept. 4, 2010 on accusations he recklessly discharged a firearm inside the limitations of municipality.
Officials learned that Alexis was cleaning a gun in his apartment when it went off and a bullet entered an upstairs apartment. No one was injured and the case was dropped.
Police in Seattle say they arrested Alexis in 2004 for shooting out the tires of a car. Detectives say the car belonged to construction workers who had parked it near a home where Alexis was staying.
When police later arrested Alexis, he claimed the construction workers had disrespected him. Alexis said it triggered an anger-fueled blackout and that he didn't even remember firing his weapon until an hour later.
According to a Seattle detective, Alexis' father said his son suffered from posttraumatic stress disorder after helping to rescue victims of the Sept. 11 attacks in New York City.
President Obama reacts to shooting
President Barack Obama was briefed on the situation and spoke about the shooting at the beginning of a news conference Monday. He was at the White House when the shooting happened.
"We still don't know all the facts, but we do know that several people have been shot and some killed. So we are confronting yet another mass shooting, and today it happened on a military installation in our nation's capital," Mr. Obama said. "It targeted our military and civilian personnel, men and women who were going to work, doing their job protecting all of us. They are patriots. And they know the dangers of serving abroad, but today they faced the unimaginable violence that they won't have expected here at home."
The president said he wanted a "seamless" investigation into the shooting and is standing with the victims and their families affected by what he called a "cowardly act."
This is the 19th mass shooting with five or more people killed since the president took office in January 2009.
This includes shootings in Binghamton, N.Y., Fort Hood, Texas, Carthage, N.C., Manchester, Conn., Tucson, Ariz., Carson City, Nev., Seal Beach, Calif., Norcross, Ga., Oakland, Calif., Seattle, Wash., Minneapolis, Minn., Aurora, Colo., Oak Creek, Wis., Newtown, Conn., and Mohawk Valley, New York.
Shootings in the past five months include Federal Way, Wash., Santa Monica, Calif., and Hialeah, Fla.
The president continues to push lawmakers to pass tougher gun control laws. Last month, he used his executive authority to close a loophole in the current background check system and bar the re-importation of surplus American military weapons.
Investigators initially seek two additional men for questioning
Authorities were initally looking into the possibility of two additional shooters but have since said they were convinced the attack was the work of a lone gunman.
In a news conference, Washington D.C. Police Chief Cathy Lanier said a white male in a khaki uniform, wearing a beret and carrying a handgun was last seen around 8:35 a.m.
The white male who was considered suspicious was identified and cleared as a suspect around 11:50 a.m.
The second person of interest was described as a black male in his 50s, standing around 5 feet 10 inches tall and weighing about 180 pounds with gray sideburns. He was last seen wearing an olive drab uniform and was reportedly armed with a long gun.
"There is no information to believe they are military personnel, but they are wearing military uniforms," Lanier said. "We have reason to believe that these people may be involved and we need to talk to them."
Law enforcement set up a massive perimeter to try and secure the area. Several blocks were closed including an entrance to the nearby Navy Yard Metro station.
Six Washington D.C. public schools and the Department of Transportation were placed on lockdown as a precaution.
People were being told to stay in their homes and out of the area as authorities searched for the remaining possible suspect. But as of 10:20 p.m. ET, officials lifted the shelter-in-place order and said there were no more suspects being sought.
A ground stop was ordered at Reagan National Airport Monday morning to help law enforcement aircraft responding to the incident. That order was lifted shortly after 10 a.m. E.T. by the FAA. Normal operations have since resumed.
Shooting happened in crowded area
The Navy Yard is a very busy area of the nation's capital, located just yards of from the Washington Nationals' baseball stadium, several federal government agencies, and only two and a half miles from the U.S. Capital.
The Navy Yard employs about 15,000 people and is the nation's oldest Navy facility, built in 1799.
The Naval Sea Systems Command, the largest of the Navy's five commands, is responsible for engineering, building, buying and maintaining ships, submarines and combat systems in the Navy's fleet.
The Navy Yard has three gates, according to its website. One is open around the clock and must be used by visitors. A second gate is only for military and civilian Defense Department employees. The other gate is for bus traffic.
DC Metropolitan Police was the primary responding agency. A spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives said they dispatched a team of approximately 20 specially, trained locally stationed special agents to the Washington Navy Yard to work with local authorities in the investigation.
The lead investigation has since been taken over by the FBI. A law enforcement official told ABC News that the FBI was questioning the suspect's father, mother and sister.
Anyone with information regarding the shooting was urged to call (800) CALL-FBI. Anyone wishing to find information on family members was urged to call (202) 433-6151.
ABC News and The Associated Press contributed to this report.