The official also says the shooter is "contained" but not yet in custody.
WASHINGTON (AP) -- Federal law enforcement officials say the man accused in a shooting rampage at the Washington Navy Yard that left at least 12 people dead has been identified as Aaron Alexis.
Alexis was killed this morning. How and when is still unclear.
The two officials spoke Monday to The Associated Press on condition of anonymity because they were not authorized to speak publicly.
One of those officials says Alexis was a 34-year-old from Texas. He is believed to have a criminal record there and to be a holder of a concealed carry weapon permit.
That official says Alexis is believed to have gotten into the Navy Yard by using someone else's identification card. It is not yet clear if that individual was an accomplice or if that person's ID card was stolen.
It is believed at least two gunmen opened fire Monday inside one of the Navy's oldest buildings, attacking office workers at a heavily guarded military facility in the heart of the nation's capital. The suspects were reportedly dressed in military-style clothing, including one who had on a beret.
The attack unfolded just a short distance from the White House and the U.S. Capitol at a former shipyard that is one of the Navy's oldest shore facilities.
The building that was targeted was the military's headquarters for Naval Sea Systems Command, which buys, builds and maintains ships, submarines and combat systems. About 3,000 people work at the headquarters, many of them civilians.
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.
It was not clear whether the witnesses on different floors were describing the same gunman.
As emergency vehicles and law enforcement officers flooded streets around the complex, a helicopter hovered overhead, nearby schools were locked down and airplanes at nearby Reagan National Airport were briefly grounded so they would not interfere with law-enforcement choppers. Less than 2 miles away, security was beefed up at the Capitol and other federal buildings, but officials said there was no known threat.
Senate officials say no one is being allowed in or out of Senate buildings while authorities search for a potential second suspect. The House remained open.
President Barack Obama mourned yet another mass shooting in the U.S. that he said took the lives of American patriots. Obama promised to make sure "whoever carried out this cowardly act is held responsible."