A man brandishing an "assault rifle," shotgun and handgun opened fire Monday inside a building at the Washington Navy Yard. The city police chief said 12 people were killed.

UPDATE: The death toll has since risen to 13 confirmed dead.

The number of injured was not clear, but some reports placed it as high as 16.

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Several sources identify the dead gunman as Aaron Alexis, 34, originally of Fort Worth, Texas. Officials say he recently began working as a civilian contractor and stole a military ID to gain access to the Navy Yard.

An individual with the same name and age as the suspect was arrested in Fort Worth in 2010 and can be seen in the mugshot to the right.

Read Aaron Alexis' Fort Worth police report

While Alexis' motive is still not clear, officials said he is a former avionics electrician with the U.S. Navy. He had been arrested at least twice previously: once in Seattle for malicious mischief, and once in Fort Worth in 2010 for discharging a firearm in public.

Alexis was carrying an ID card belonging to Rollie Chance, who was placed on administrative leave last October. Chance told officials he does not know Alexis.

"He lived with me three years," a Texas man named Nutpisit Suthamtewakul told the Star-Telegram, calling Alexis his "best friend." He said Alexis had been working for a computer contractor. "I don't think he'd do this," said Suthamtewakul. "He has a gun but I don't think he's that stupid. He didn't seem aggressive to me."

Reports that more than one shooter was involved have not been confirmed. Police continue to seek a black male with gray sideburns for questioning.

Earlier in the day, Washington police issued lookouts for two people they described as suspects — a 50-year-old black man with a rifle, wearing a drab olive military uniform, and a white man with a pistol, wearing a short-sleeved, khaki uniform and a beret.

Minutes later, police said on Twitter that one of the two had been identified and was not a suspect or person of interest.

WATCH: Live coverage of shooting rampage at D.C. Navy base

The FBI is now taking the lead in the investigation.

The Bureau of Alcohol, Tobacco, Firearms and Explosives sent a team of about 20 special agents to the scene, a law enforcement official told CNN. The team was the same group that helped apprehend Boston Marathon bombing suspect Dzhokhar Tsarnaev, the official said.

WNEW: Photos from scene of Navy Yard shooting

The Pentagon Force Protection Agency has "increased its security posture, not out of a specific threat, but as a proactive, precautionary measure related to the ongoing incident at the Navy Yard."

This means more officers at Pentagon entrances armed with automatic weapons, according to Lt. Col. Tom Crosson, a Pentagon spokesman.

The U.S. Capitol Police are also conducting "enhanced security operations" on the U.S. Capitol grounds, though there is no known threat to the complex at this point.

Eyewitnesses described the gunman as a black male dressed in all black, carrying an "assault rifle" and a double-barreled shotgun.

Terrie Durham, who works at the Naval Sea Systems Command building, said the gunman appeared to be wearing dark fatigues. “He was tall. He appeared to be dark-skinned,” she told WJLA-TV.

“He was a tall black guy,” said her co-worker, Todd Brundidge, who is black. “He didn’t say a word. He aimed the gun and fired our way, 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 the TV station. "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."

Rick Mason, a program management analyst who is a civilian with the U.S. Navy, told the Associated Press that a gunman was shooting from a fourth floor overlook in the hallway outside his office. He said the gunman was aiming at people in the building’s first floor cafeteria.

Patricia Ward, who works at the Navy Yard, said she had just gotten breakfast in the cafeteria when she heard “three gunshots, pow-pow-pow, straight in a row.”

“All of the people that were in the cafeteria, we all panicked, and we were trying to decide which way we were going to run out,” she said. “I just ran.”

Ward said the building is heavily secured at all times.

"You need a car to enter the building. It's very hard to get in without a card."

"Being with the incident that happened today, not secure enough for me," Ward added.

Police spokesman Chris Kelly described the suspect as an adult male, about 6 feet tall with a bald head and medium complexion, dressed in a black top and black jeans.

A naval security guard was among those shot and was hit in both legs, U.S. military officials said. Washington city police told WRC, the NBC affiliate in Washington, that one of their officers was also among those shot. It was not clear how many of the others shot were civilian and how many were military.

The Navy said on its Twitter feed that shots were fired at 8:20 a.m. ET at the headquarters of the Naval Sea Systems Command, part of the Washington Navy Yard.

The Washington Navy Yard says on its website that it's the Navy's oldest shore establishment and the largest of the Navy's five system commands. It's home to the chief of Naval Operations and is also headquarters for the Naval Historical Center and numerous naval commands. It builds, buys and maintains the Navy's ships and submarines and their combat systems.

The Navy Yard is on lockdown and a “shelter in place” order has been issued, the Navy says. Approximately 3,000 people work at the facility.

Tim Hogan, a spokesman for Rep. Steven Horsford of Nevada, posted photos to his Twitter account of people tending to at least one person down on a street corner.

President Barack Obama lamented that the U.S. is "confronting another mass shooting" and called for a seamless investigation involving federal agencies and the military.

"I've been briefed by my team on the situation. 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. 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."

Planes have resumed flying out of nearby Reagan National Airport after a ground stop was imposed by the FAA due to the ongoing situation at Navy Yard earlier this morning, a Metropolitan Washington Airports Authority spokeswoman told ABC News.

The rampage was the deadliest mass shooting in the United States since the massacre at Sandy Hook Elementary School in Newtown, Conn., last December, and the worst at a military installation since 13 people were killed at Fort Hood, Texas, in 2009.