How Aaron Alexis's U.S. navy yard shooting unfolded

A man opened fire at the Naval Sea Systems Command building in the Washington Navy Yard on Monday in a shooting that left 13 people dead, including the gunman, not far from the U.S. Capitol and White House, officials said.

• Washington residents post from scene of Navy Yard shooting
• Aaron Alexis suffered PTSD, father told police

Below is a timeline of the shooting according to witnesses and police:

8:20 a.m. ET:A gunman, later identified as 34-year-old Aaron Alexis of Fort Worth, Texas, enters the five-story building. He shoots at people from the fourth-floor of an atrium into the busy, first-floor cafeteria and at people fleeing their offices in the ensuing chaos.

Fire alarms sound and alternate with security announcements. Workers barricade themselves in offices and conference rooms.

8:22-8:23 a.m.:Washington city and U.S. Park Police officers arrive to join base security in fighting the gunman. There are already victims.

8:27 a.m.:SWAT police enter the building as officers and emergency personnel surround the Navy Yard. Intermittent gunfire sounds throughout the site.

8:35-8:40 a.m.:Police see a man they believe might be a second shooter, armed with a handgun. Police later say the man has been identified and is not a suspect.

Police say a third man, described as black and armed with a rifle, is a possible suspect in the rampage.

Shortly after 9 a.m.:Gunfire dies down. Police swarm Navy Yard to search buildings and remove people from shelters. Helicopters remove victims from the roof of Sea Systems Command building.

About 9:30 a.m.:Ambulances take victims to hospitals.

10 a.m.:The Federal Aviation Administration lifts brief ban on departures from Reagan National Airport because of law enforcement helicopters flying over the Navy Yard.

About 11:30 a.m.:Police say a gunman is dead.

About 3 p.m.:Senate workers ordered to shelter in place. Senate cancels session. Nationals Major League Baseball team cancels night game with Atlanta Braves.

Roads, highways and some schools were shut down during the day. Some personnel at the base will remain locked down into the evening.

About 6:30 p.m.:With the capital on high alert, a man throws firecrackers over the White House's north fence line. He is arrested.