Chronology of the key events of Sept. 11, 2001 (all times Eastern):
8 a.m. -- American Airlines Flight 11, Boeing 767 with 92 people on board, takes off from Boston's Logan International Airport for Los Angeles.
8:14 a.m. -- United Air Lines Flight 175, Boeing 767 with 65 people on board, takes off from Boston's Logan airport for Los Angeles.
8:21 a.m. -- American Airlines Flight 77, Boeing 757 with 64 people on board, takes off from Washington Dulles International Airport for Los Angeles.
8:40 a.m. -- Federal Aviation Administration notifies North American Aerospace Defense Command's Northeast Air Defense Sector about suspected hijacking of American Flight 11.
8:41 a.m. -- United Air Lines Flight 93, Boeing 757 with 44 people on board, takes off from Newark International Airport for San Francisco.
8:43 a.m. -- FAA notifies NORAD's Northeast Air Defense Sector about suspected hijacking of United Flight 175.
8:46 a.m. -- American Flight 11 crashes into north tower of World Trade Center.
9:03 a.m. -- United Flight 175 crashes into south tower of World Trade Center.
9:08 a.m. -- FAA bans all takeoffs nationwide for flights going to or through New York airspace.
9:17 a.m. -- FAA closes down all New York City-area airports.
9:21 a.m. -- All bridges and tunnels into Manhattan closed.
9:24 a.m. -- FAA notifies NORAD's Northeast Air Defense Sector about suspected hijacking of American Flight 77.
9:26 a.m. -- FAA bans takeoffs of all civilian aircraft.
9:31 a.m. -- In Sarasota, Fla., President Bush calls crashes an "apparent terrorist attack on our country."
9:40 a.m. (approx.) -- American Flight 77 crashes into Pentagon.
9:45 a.m. -- FAA orders all aircraft to land at nearest airport as soon as practical. More than 4,500 aircraft in air at the time. This is the first time in U.S. history that nationwide air traffic is suspended.
9:48 a.m. -- U.S. Capitol and White House's West Wing evacuated.
9:57 a.m.-- President Bush leaves Florida.
9:59 a.m. -- South tower of World Trade Center collapses.
10:07 a.m. (approx.) -- United Flight 93 crashes in a field in Shanksville, Pa., southeast of Pittsburgh.
10:28 a.m. -- North tower of World Trade Center collapses.
10:50 a.m. -- New York's primary elections, scheduled for Sept.11, are postponed.
10:56 a.m. -- Palestinian leader Yasser Arafat speaks in Gaza: "First of all, I am offering my condolences, the condolences of the Palestinian people, to their American President, President Bush, to his government, to the American people, for this terrible time. We are completely shocked, completely shocked. Unbelievable."
11 a.m. -- New York City Mayor Rudolph W. Giuliani orders the evacuation of lower Manhattan south of Canal Street.
11:04 a.m.-- The United Nations is fully evacuated.
12:04 p.m. -- Los Angeles International Airport, the destination of three of the hijacked airplanes, is closed and evacuated.
12:15 p.m. -- San Francisco International Airport, the destination of United Airlines Flight 93, which crashed in Pennsylvania, is closed and evacuated.
12:30 p.m. -- The FAA says 50 flights are in U.S. airspace, but none are reporting any problems.
1:04 p.m. -- From Barksdale Air Force base in Louisiana, Bush announces U.S. military on high-alert worldwide: "Make no mistake: The United States will hunt down and punish those responsible for these cowardly acts."
1:37 p.m. -- Bush leaves Barksdale for Offutt Air Force Base, near Omaha, Neb.
2:51 p.m. -- Navy dispatches missile destroyers to New York, Washington.
3:07 p.m. -- Bush arrives at U.S. Strategic Command at Offutt Air Force Base in Neb.
4:36 p.m. -- Bush leaves Offutt Air Force Base aboard Air Force One to return to Washington.
5:25 p.m. -- The empty, 47-story Seven World Trade Center collapses.
7 p.m. -- Bush arrives at the White House.
8:30 p.m. -- Bush addresses nation and vows to "find those responsible and bring them to justice."
The Associated Press contributed to this report.