WASHINGTON (CNN) -- Thirteen people were killed Monday after a believed single shooter opened fire in a rampage at a Navy yard in the nation's capital, putting government buildings on lockdown and sending police SWAT teams rushing to the scene. The 13 dead include 34-year-old suspect Aaron Alexis.
Authorities have identified seven of the 12 DC shootings victims. They are: Michael Arnold, 59. Sylvia Frasier, 53. Kathy Gaarde, 62. John Roger Johnson, 73. Frank Kohler, 50. Kenneth Bernard Proctor, 46. Vishnu Pandit, 61. All of the victims killed are described as civilians or contractors; none of the victims killed were military personnel, authorities said. At a Monday night press conference authorities stated they believed that none of the victims were active-duty military, but that will be confirmed when the other five names are released.
Besides the 13 people who were killed, eight people were injured in Monday morning's shooting, Washington Mayor Vincent Gray told reporters Monday night. Three of those were injured by gunfire, and the others had other types of injuries, such as contusions and chest pain. Earlier Monday night, Navy Vice Adm. William D. French said 14 people were injured.
Washington police are confident that Alexis was the only person involved in Monday morning's shooting at the Washington Navy Yard, and they are lifting a shelter-in-place order for residents who live nearby, Police Chief Cathy Lanier said Monday night. Authorities have said suspect Alexis was killed after an encounter with security.
President Obama called the shooting a "cowardly act" that targeted military and civilians serving their country.
"They know the dangers of serving abroad," he said, "but today they faced the unimaginable violence that we wouldn't have expected here at home."
The violence started unfolding at 8:20 a.m. when several shots were fired inside the headquarters for Naval Sea Systems Command in southeast Washington.
Two witnesses told CNN affiliate WJLA-TV that they heard a fire alarm go off in the building where they worked, then saw a 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."
People frantically tried to run out of the building, Brundidge said.
"Everyone was going down the stairs. They were pushing. They were shoving. People were falling down," he told WJLA. "As we came outside, people were climbing the wall trying to get over the wall to get out. .... It was just crazy."
The injured included a Washington police officer who has been hospitalized, and a base security guard officer, said Metropolitan Police Department spokeswoman Saray Leon.
Three people, including the D.C. police officer, were admitted to MedStar Washington Hospital Center with multiple gunshot wounds. They are expected to survive, chief medical officer Janis Orlowski told reporters.
One person was pronounced dead at George Washington University Hospital, according to Dr. Babak Sarani, chief of trauma and acute care there.
Meanwhile, at the Navy yard, helicopters hovered overhead. In one chopper, there appeared to be a police sniper peering out, with a scope at the ready.
The Bureau of Alcohol, Tobacco, Firearms and Explosives sent a team of about 20 special agents to the scene, a law enforcement official said. The team was the same group that helped apprehend Boston Marathon bombing suspect Dzhokhar Tsarnaev, the official said.
Emergency personnel, the FBI, U.S. Capitol Police and local D.C. police responded to the shooting, shutting down traffic in the area on the District's south side along the Anacostia River. Some people were evacuated, and others sheltered in place.
Paul Williams, who works at a nearby nonprofit, was headed to his office when he witnessed panic at the Navy yard.
"I heard four rapid bangs -- bang, bang, bang, bang," he said.
At first, he thought it was construction noise, but less than a minute later, he saw hundreds of people coming toward him.
"I didn't know what was happening. I just ran with them," Williams said. "Everyone seemed scared. People were crying. People were being consoled and calling loved ones and family."
Security was stepped up at the Pentagon. At least eight schools were on lockdown as a precaution, the Washington public schools said.
Air traffic to Reagan National Airport in northern Virginia, the closest airport to downtown Washington, was suspended but later resumed, the Federal Aviation Administration said.
The headquarters -- the workplace for about 3,000 people -- is the largest of the Navy's five system commands. It has a fiscal year budget of nearly $30 billion.
"With a force of 60,000 civilian, military and contract support personnel, NAVSEA engineers, builds, buys and maintains the Navy's ships and submarines and their combat systems," the Navy said.
The Washington Navy Yard -- the Navy's oldest land establishment -- was created in 1799 following an act of Congress, according to the Naval History and Heritage Command. Originally envisioned as a shipbuilding and fitting facility on the Anacostia River, it serviced some of the Navy's most famous early vessels, including the USS Constitution.
Burned during the War of 1812, the Navy Yard was transformed into a center for ordnance and technological development. The facility was the world's largest ordnance plant during World War II, but its military role steadily diminished during the Cold War era.
Today, the Navy Yard includes the headquarters of Naval District Washington and is home to a naval museum. The area around the facility has been marked in recent years by significant commercial and residential revitalization.
CNN's Barbara Starr reported from Washington, and CNN's Catherine E. Shoichet reported from Atlanta. CNN's Tom Cohen, Dan Merica, Larry Shaughnessy, Brian Todd, Alan Silverleib, Eliott C. McLaughlin, Joe Sterling, Paul Courson and Evan Perez contributed to this report.