Washington (CNN) -- 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.
10:34 a.m. -- Metro police spokeswoman Saray Leon denies media reports about three suspects possibly involved, saying authorities were looking for one suspect.
10:40 a.m. Counterterrorism officials have briefed President Barack Obama "several times" about the shooting, a White House official says.
10:46 a.m. At least 10 people have been injured, according to police. Navy issues statement saying there are "several injuries with reports of fatalities."
10:55 a.m. -- SWAT team moves through main gate and helps get people out of the building.
11:07 a.m. -- Law enforcement official reports the federal Bureau of Alcohol Tobacco, Firearms and Explosives emergency response team is sent to the scene. It is the same one that helped Boston police apprehend Dzhokhar Tsarnaev. The official notes the team includes approximately 20 specially trained, locally stationed agents.
Confusion over shooters
11:29 a.m. -- Ed Zeigler, Director of Public Affairs for Naval District Washington, tells CNN two shooters "are down" at the Navy Yard. Zeigler cannot confirm if the shooters are in custody or dead.
Zeigler also tells CNN there have been "multiple deaths" at the Navy Yard. Zeigler says law enforcement is still trying to clear Building 197 to determine if there are any more shooters.
11:31 a.m.: Helicopter hover over the scene, and it appears one chopper contains a police sniper.
11:50 a.m. -- A senior Navy official tells CNN's Barbara Starr that a suspected shooter is dead.
The suspect was identified as Aaron Alexis, 34, a former Navy reservist and a current military contractor, the Washington FBI Field Office told CNN. His identity was confirmed by fingerprints and a picture ID card.
11:38 a.m. -- Janice Orlowski, chief medical officer at MedStar Washington Hospital Center, notes that the weapon used to shoot the victims "had to be a semi-automatic because they're talking about gunshots that they heard in rapid succession."
12:14 p.m.: Police say they are looking for two other possible suspects. One was described as a white male in a khaki tan military uniform with a handgun. Authorities also were on the lookout for a black male, about 50, who may have had a long gun. That person was wearing an olive, drab colored, possible military style uniform, Lanier said.
The white male was later cleared of any involvement, while the search continued for the second man, authorities said.
2 p.m.: Washington Mayor Vincent Gray said 12 people were killed. He later added a 13th fatality, including the main suspect.
7 p.m.: FBI investigators rule out additional shooters but Metropolitan Police still sought one person to discern if there was any involvement.
Alan Silverleib, Evan Perez, Brian Todd, Barbara Starr and Tom Cohen contributed to this report.