WASHINGTON (AP) -- As many as three gunmen opened fire Monday inside the Washington Navy Yard, attacking office workers at a heavily guarded military facility in the heart of the nation's capital. At least 12 people were killed. One of the gunmen was dead, and police were searching for two other men believed to have joined in the attack less than 4 miles from the White House. The suspects were reportedly disguised in military-style clothing, including one carrying a handgun and wearing a tan Navy-style uniform and a beret, D.C. police chief Cathy Lanier said. The other was said to carrying a long gun, wearing olive green garb. "The big concern for us right now is, we potentially have two other shooters that we have not located," Lanier said. It was not immediately clear whether the number of dead included a gunman. The attack unfolded less than 4 miles from the White House at a former shipyard that is one of the Navy's oldest shore facilities. The area that was targeted, known as Building 197, was part of 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. A short distance 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." Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a long hallway on the third floor. The gunman was wearing all blue, he said. "He just turned and started firing," Brundidge said. Terrie Durham, an executive assistant with the same agency, said she also saw the gunman firing toward her and Brundidge. "He aimed high and missed," she said. "He said nothing. As soon as I realized he was shooting, we just said, 'Get out of the building.'" Rick Mason, a civilian program-management analyst for the Navy, said a gunman was shooting from the overlook in the hallway outside his office. Shortly after the gunfire, Mason said, someone on an overhead speaker told workers to seek shelter and later to head for the gates at the complex. Patricia Ward, a logistics-management specialist, said she was in the cafeteria getting breakfast. "It was three gunshots straight in a row -- pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward told reporters several blocks away from the Navy Yard. Ward said security officers started directing people out of the building with guns drawn. One person died at George Washington University Hospital of a single gunshot wound to the left temple, said Dr. Babak Sarani, director of trauma and acute care surgery. A police officer and two civilian women were in critical condition at Washington Hospital Center, said Janis Orlowski, the hospital's chief operating officer. Orlowski said the police officer was in the operating room with gunshot wounds to the legs. The police chief said he was wounded when he engaged the shooter. It wasn't clear if he shot at the gunman. One woman had a gunshot wound to the shoulder. The other had gunshot wounds to the head and hand. Anxious relatives and friends of those who work at the complex waited to hear from loved ones. Tech Sgt. David Reyes, who works at Andrews Air Force Base, said he was waiting to pick up his wife, Dina, who was under lockdown in a building next to where the shooting happened. She sent him a text message about being on lockdown. "They are under lockdown because they just don't know," Reyes said. "They have to check every building in there, and they have to check every room and just, of course, a lot of rooms and a lot of buildings." Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. Only security personnel were allowed to be armed on the campus. Mason, the program management analyst for the Navy, said there are multiple levels of security to reach his office where he heard gunfire. Everyone must show their building IDs to get through a main gate, and at the building entrance, everyone must swipe their badges to pass through either a door or gate, depending on the entrance. That "makes me think it might have been someone who works here," he said. The Navy Yard has three gates, according to its website. One is open around the clock and must be used by visitors. A second gate is only for military and civilian Defense Department employees. The other gate is for bus traffic. The Navy Yard is part of a fast-growing neighborhood on the banks of the Anacostia River in southeast Washington, just blocks from the Nationals Park baseball stadium.