8:45 a.m.: A hijacked passenger jet, American Airlines Flight 11 out of Boston, Massachusetts, crashes into the north tower of the World Trade Center, tearing a gaping hole in the building and setting it afire.

9:03 a.m.: A second hijacked airliner, United Airlines Flight 175 from Boston, crashes into the south tower of the World Trade Center and explodes. Both buildings are burning.

9:17 a.m.: The Federal Aviation Administration shuts down all New York City area airports.

9:21 a.m.: The Port Authority of New York and New Jersey orders all bridges and tunnels in the New York area closed.

9:30 a.m.: President Bush, speaking in Sarasota, Florida, says the country has suffered an "apparent terrorist attack."

9:40 a.m.: The FAA halts all flight operations at U.S. airports, the first time in U.S. history that air traffic nationwide has been halted.

9:43 a.m.: American Airlines Flight 77 crashes into the Pentagon, sending up a huge plume of smoke. Evacuation begins immediately.

9:45 a.m.: The White House evacuates.

9:57 a.m.: George Bush departs from Florida.

10:05 a.m.: The south tower of the World Trade Center collapses, plummeting into the streets below. A massive cloud of dust and debris forms and slowly drifts away from the building.

10:08 a.m.: Secret Service agents armed with automatic rifles are deployed into Lafayette Park across from the White House.

10:10 a.m.: A portion of the Pentagon collapses.

10:10 a.m.: United Airlines Flight 93, also hijacked, crashes in Somerset County, Pennsylvania, southeast of Pittsburgh.

10:13 a.m.: The United Nations building evacuates, including 4,700 people from the headquarters building and 7,000 total from UNICEF and U.N. development programs.

10:22 a.m.: In Washington, the State and Justice departments are evacuated, along with the World Bank.

10:24 a.m.: The FAA reports that all inbound transatlantic aircraft flying into the United States are being diverted to Canada.

10:28 a.m.: The World Trade Center's north tower collapses from the top down as if it were being peeled apart, releasing a tremendous cloud of debris and smoke.

10:45 a.m.: All federal office buildings in Washington are evacuated.

10.46 a.m.: U.S. Secretary of State Colin Powell cuts short his trip to Latin America to return to the United States.

10.48 a.m.: Police confirm the plane crash in Pennsylvania.

10:53 a.m.: New York's primary elections, scheduled for Tuesday, are postponed.

10:54 a.m.: Israel evacuates all diplomatic missions.

10:57 a.m.: New York Gov. George Pataki says all state government offices are closed.

11:02 a.m.: New York City Mayor Rudolph Giuliani urges New Yorkers to stay at home and orders an evacuation of the area south of Canal Street.

11:16 a.m.: CNN reports that the Centers for Disease Control and Prevention is preparing emergency-response teams in a precautionary move.

11:18 a.m.: American Airlines reports it has lost two aircraft. American Flight 11, a Boeing 767 flying from Boston to Los Angeles, had 81 passengers and 11 crew aboard. Flight 77, a Boeing 757 en route from Washington's Dulles International Airport to Los Angeles, had 58 passengers and six crew members aboard. Flight 11 slammed into the north tower of the World Trade Center. Flight 77 hit the Pentagon.

11:26 a.m.: United Airlines reports that United Flight 93, en route from Newark, New Jersey, to San Francisco, California, has crashed in Pennsylvania. The airline also says that it is "deeply concerned" about United Flight 175.

11:59 a.m.: United Airlines confirms that Flight 175, from Boston to Los Angeles, has crashed with 56 passengers and nine crew members aboard. It hit the World Trade Center's south tower.

12:04 p.m.: Los Angeles International Airport, the destination of three of the crashed airplanes, is evacuated.

12:15 p.m: San Francisco International Airport is evacuated and shut down. The airport was the destination of United Airlines Flight 93, which crashed in Pennsylvania.

12:15 p.m.: The Immigration and Naturalization Service says U.S. borders with Canada and Mexico are on the highest state of alert, but no decision has been made about closing borders.

12:30 p.m.: The FAA says 50 flights are in U.S. airspace, but none are reporting any problems.

1:04 p.m.: Bush, speaking from Barksdale Air Force Base in Louisiana, says that all appropriate security measures are being taken, including putting the U.S. military on high alert worldwide. He asks for prayers for those killed or wounded in the attacks and says, "Make no mistake, the United States will hunt down and punish those responsible for these cowardly acts."

1:27 p.m.: A state of emergency is declared by the city of Washington.

1:44 p.m.: The Pentagon says five warships and two aircraft carriers will leave the U.S. Naval Station in Norfolk, Virginia, to protect the East Coast from further attack and to reduce the number of ships in port. The two carriers, the USS George Washington and the USS John F. Kennedy, are headed for the New York coast. The other ships headed to sea are frigates and guided missile destroyers capable of shooting down aircraft.

1:48 p.m.: Bush leaves Barksdale Air Force Base aboard Air Force One and flies to an Air Force base in Nebraska.

2:00 p.m.: Senior FBI sources tell CNN they are working on the assumption that the four airplanes that crashed were hijacked as part of a terrorist attack.

2:30 p.m.: The FAA announces there will be no U.S. commercial air traffic until noon EDT Wednesday at the earliest.

2:49 p.m.: At a news conference, Giuliani says that subway and bus service are partially restored in New York City. Asked about the number of people killed, Giuliani says, "I don't think we want to speculate about that -- more than any of us can bear."

3:55 p.m.: Karen Hughes, a White House counselor, says the president is at an undisclosed location, later revealed to be Offutt Air Force Base in Nebraska, and is conducting a National Security Council meeting by phone. Vice President Dick Cheney and National Security Adviser Condoleezza Rice are in a secure facility at the White House. Defense Secretary Donald Rumsfeld is at the Pentagon.

3:55 p.m.: Giuliani now says the number of critically injured in New York City is up to 200 with 2,100 total injuries reported.

4:00 p.m: CNN National Security Correspondent David Ensor reports that U.S. officials say there are "good indications" that Saudi militant Osama bin Laden, suspected of coordinating the bombings of two U.S. embassies in 1998, is involved in the attacks, based on "new and specific" information developed since the attacks.

4:06 p.m.: California Gov. Gray Davis dispatches urban search-and-rescue teams to New York.

4:10 p.m.: Building 7 of the World Trade Center complex is reported on fire.

4:20 p.m.: U.S. Sen. Bob Graham, D-Florida, chairman of the Senate Intelligence Committee, says he was "not surprised there was an attack (but) was surprised at the specificity." He says he was "shocked at what actually happened -- the extent of it."

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