8:05 a.m.: A second hijacked airliner hit Tower 2, sending flames and smoke billowing from that part of the World Trade Center.
8:15 a.m.: An unmanned surveillance plane shot down over Iraq.
8:55 a.m.: President Bush addresses nation, promises to "hunt down the folks who committed this act." Bush was in Sarasota, Fla., and was enroute back to Washington.
9 a.m.: In Pennsylvania, a Boeing 757 crashes just north of the Somerset County Airport. The fate of those aboard was not immediately known and it was not clear if the crash was related to the disasters elsewhere. The United Airlines flight originated from Newark, N.J., enroute to San Francisco.
9 a.m.: The 110-story southern tower of the World Trade Center collapses with a roar and a huge cloud of smoke.
9:30 a.m.: The second World Trade Center tower falls, covering lower Manhattan in heaps of rubble and broken glass. Firefighters trapped in the rubble radioed for help.
9 a.m.: An aircraft crashes on a helicopter landing pad near the Pentagon, and the White House, the Pentagon and the Capitol are evacuated.
11:06 a.m.: United Airlines confirms second United plane crashing, location not given.
Authorities went on alert from coast to coast, halting all air traffic, evacuating high-profile buildings and tightening security at strategic installations. The Situation Room at the White House was in full operation.
Evacuations were ordered at the United Nations in New York and at the Sears Tower in Chicago. Los Angeles mobilized its anti-terrorism division, and security was intensified around the naval facilities in Hampton Roads, Va. Trading on Wall Street was suspended.