For more than 30 agonizing minutes, Aaron Alexis stalked his human prey with a 12-gauge shotgun, eluding several attempts by law enforcement officers to gun him down, police said Tuesday.
Details emerging about the chaotic morning of violence at the Washington Navy Yard paint a picture of man with a calculated plan bent on indiscriminately killing as many people as he could.
He slayed 12 before police ended his life in a gunbattle.
HOW DID IT HAPPEN?: Contractors stunned by security clearance
Federal investigators believe Alexis, an hourly computer tech employed by a Navy contractor, cleared a security checkpoint at the Navy Yard's entrance with his contractor identification badge. A federal official knowledgeable about the investigation but not authorized to speak publicly said investigators believe Alexis carried a disassembled 12-gauge shotgun into building 197 and reassembled it in a men's room before he went to an upper floor and began shooting into the building's central atrium.
The first 911 call came in to a police dispatcher between 8:15 a.m. and 8:20 a.m., D.C. Metropolitan Police Chief Cathy Lanier said. A tweet from the U.S. Navy confirmed a report of three shots fired at 8:20 a.m. Within two minutes, Lanier said, two police units responded. Within four minutes, five to seven additional police units had entered the Navy Yard gates and were responding to a flurry of calls from several buildings reporting gunshots, she said.
QUESTIONS EMERGE: Why was Alexis able to buy guns?
Seven minutes after the initial call, police officers outside Building 197 heard shots and immediately entered the building, Lanier said.
As alarms sounded and workers fled, federal investigators believe, Alexis ran down a flight of stairs where he confronted and shot a security officer, took the officer's handgun and returned to the balcony overlooking the atrium, where he continued to shoot, the federal official said.
At some point, the federal official said, Alexis again left the overlook and confronted a victim described as a maintenance person or building staffer. Alexis allegedly shot that person and returned one last time to the overlook, where he was killed in a confrontation with police.
MORE ON SHOOTER: He reported hearing voices through walls
Police locked down the Navy base. Around 9 a.m., workers in other buildings were told to stay in their offices with the doors closed. In one neighboring building, police evacuated the workers to the lowest floor of a concrete parking structure at 10:30 a.m. Workers said they remained there for two hours before they were moved to a food court. Employees were allowed to leave around 3 p.m., but those who drove could not retrieve their cars. Alexis' car was found in the Navy Yard parking lot, police said.
Lanier said law enforcement officials engaged in "multiple" gunbattles with Alexis.
"I don't have the exact time of the final engagement," Lanier said. She said the shooting lasted "more than a half-hour from start to finish," but less than an hour. Reporters near the scene heard the last gunshots before 10 a.m.
Lanier said workers were asked to "shelter in place" while police worked to determine whether additional shooters were on the scene. Police initially sought two other men. Both men had been cleared by Monday night.
"We had information that we could not dismiss," she said. "We erred on the side of caution."