WASHINGTON -- Police said at least 12 people have died in the shootings at the Washington Navy Yard.D.C. Police Chief Cathy Lanier said during a news conference Monday that 12 people were confirmed dead. Lanier said people are being told to stay in their homes and out of the area as authorities search for two other possible suspects. One of the shooters has died. The police chief said officers are searching for two other people with firearms wearing military-style uniforms. She said there is no indication of a possible motive at this time. As many as three 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. One of the gunmen was dead, and police were searching for two other men believed to have joined in the attack at the Washington Navy Yard. 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. 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."