9:03 a.m.: A second airliner crashes into its twin south tower, causing a devastating explosion.

9:10 a.m.: In Florida, President Bush is reading to children in a classroom when his chief of staff, Andrew Card, whispers news of the attacks into his ear.

9:20 a.m.: The FBI investigates reports of planes being hijacked before the World Trade Center crashes.

9:29 a.m.: EDT: First reports of casualties indicate that at least six people were killed, with at least 1,000 injured.

9:30 a.m.: Bush declares: "We have had a national tragedy. Two airplanes have crashed into the World Trade Center in an apparent terrorist attack on our country."

9:43 a.m.: Abu Dhabi television reports it received a call from the Democratic Front for the Liberation of Palestine, claiming responsibility for crashing two planes into the WTC. However, leading officials later deny the claim.

9:43 a.m.: Another plane crashes into the Pentagon in Washington. The nerve center of the U.S. military bursts into flames and a portion of one side of the five-sided structure collapses.

9:48 a.m.: The White House and the Capitol are evacuated amid further threats.

9:49 a.m.: All airports across the U.S. shut down.

9:50 a.m.: The south tower of the World Trade Center collapses.

10:00 a.m.: United Airlines Flight 93, en route from Newark, N.J., to San Francisco, crashes near Pittsburgh. The crash site was 85 miles northwest of Camp David.

10:29 a.m.: The north tower of the World Trade Center collapses.

12:33 p.m.: United Airlines confirms a second of its planes has crashed at an unknown location.

12:39 p.m.: Bush makes second statement, vowing to hunt down and punish those responsible.

2:48 p.m.: New York Mayor Rudy Giuliani says the eventual death toll from Tuesday's attack may be "more than any of us can bear."

2:51 p.m.: The Navy dispatches missile destroyers and other equipment to New York and Washington.

4:25 p.m.: The American Stock Exchange, the Nasdaq and the New York Stock Exchange say they will remain closed on Wednesday.

4:30 p.m.: Bush leaves Offutt Air Force Base aboard Air Force One to return to Washington, where he will make a nationally televised address.

5:25 p.m.: World Trade Center 7 collapses.

8:30 p.m.: Bush addresses the nation. "These acts of mass murder were intended to frighten our nation into chaos and retreat," he said. "But they have failed; our country is strong."