A mass shooting at the Washington Navy Yard on Monday left at least 12 people dead, plus a suspect, in what Metropolitan Police Chief Cathy Lanier called "one of the worst things we've seen."
Gunshots shattered a quiet late summer morning at the historic military post inside the largest of the Navy's five commands.
What follows is a timeline of events as they unfolded compiled from law enforcement, witnesses, Twitter and CNN and affiliate reporting.
* Suspect, who authorities believe was staying at a Washington hotel, drove onto the post with his military contractor identification and parked. He walked a short distance into the Naval Sea Systems Command and made his way to an overlook above atrium and started shooting down, a U.S. law enforcement official told CNN.
He was armed with an AR-15, a rifle and a semi-automatic Glock. It is believed he used AR-15 for most of the shooting, the official said.
*Just after 8:15 a.m. ET.: First frantic calls to police are received about shots at Building 197.
* Within two to three minutes Metropolitan Police officers were on the scene, said Police Chief Cathy Lanier.
* Internal security had already identified and engaged the suspect. We already had victims down at that point, Lanier said.
* A Metropolitan Police officer and a base security officer were hurt.
* Officers enter the building and communicate to commanders what they saw: multiple victims and gunfire, Lanier said.
* Within seven minutes we had active shooter teams, moving through the building, Lanier said.
* She described a sporadic gun battle -- "mulitple engagements" - that witnesses said lasted 40 minutes or so.
* Police helicopters converge on the scene, prompting the Federal Aviation Administration to temporarily halt flights at Reagan National airport to clear the air space around the city.
* 10:05 a.m.: Navy confirms via Twitter several injuries with reports of fatalities.
What witnesses saw:
* Fire alarm blares through facility where shooter reported inside the building, according to witness Terrie Durham.
* Wardens yelled for everyone to get out as fast as possible.
* Durham and Todd Brundidge were part of a group that rushed into a hallway toward the exit. Then they saw a man with a rifle. "We noticed him down the hall and he stepped around the corner and we heard shots," Brundidge told CNN affiliate WJLA. What seemed like two or three seconds passed, and then "he aimed his gun at us and then he fired at least two or three shots."
* Durham said the gunman was too far away to see his face, but "we could see him with the rifle and he raised and aimed at us and fired." The shots hit high on the wall, and the group she was with ran out of the building.
* Patricia Ward said she heard three shots "pow, pow, pow." She said she heard three more shots from the direction of the cafeteria 30 seconds later.
* Cmdr. Tim Juris said someone was shot right in front of him.
"We were just standing here ... having a conversation and we heard two more gunshots and he went down. That's when I ran," Juris said.
SWAT team on scene
10:06 a.m. -- Chris Kelly, Metro police spokesman, tells CNN there are 10 victims Kelly tells CNN the main suspect was an adult black male, 6 feet, bald, medium complexion and wearing black clothing - black top and black jeans.