“The big concern for us right now is that we potentially have two other shooters that we have not located at this point,” Washington Police Chief Cathy Lanier told reporters.
Multiple people were killed in the shooting rampage, Lanier said, which sent police in SWAT team gear swarming to the scene Monday morning.
Hours later, Lanier said authorities were still looking for a white man and a black man in military-style clothing who could be connected to the shooting.
“We have no information to believe that either of those folks are military personnel, but we do have information that those individuals are wearing military-style uniforms,” she said.
Earlier Monday, Ed Zeigler, director of public affairs for Naval District Washington, told CNN that two suspected shooters were “down,” but a senior Navy official could not confirm whether there was more than one gunman.
“We will do everything in our power to make sure whoever carried out this cowardly act is held responsible,” President Barack Obama said, adding the shooting “targeted military and civilian personnel.”
“We still don’t know all the facts. But we do know that several people have been shot and some have been killed,” he said. “So we are confronting yet another mass shooting. And today it happened on a military installation in our nation’s capital.”
Earlier, authorities said a gunman dressed in all black fired shots inside the Navy Yard, injuring at least 10 people.
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 incident started unfolding at 8:20 a.m. when several shots were fired inside the headquarters for Naval Sea Systems Command in southeast Washington.
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.
Police spokesman Chris Kelly earlier described a suspect as an adult male, about 6 feet tall with a bald head and medium complexion, dressed in a black top and black jeans.
Two witnesses told CNN affiliate WJLA 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 of the spaces. It was just crazy.”
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 are being evacuated, and others are sheltering in place.
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, Joe Sterling, Dan Merica, Paul Courson and Evan Perez contributed to this report.