8:45 a.m. (all times are EDT): 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.: 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 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 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.

4:30 p.m.: The president leaves Offutt Air Force Base in Nebraska aboard Air Force One to return to Washington.

5:15 p.m.: CNN Military Affairs Correspondent Jamie McIntyre reports fires are still burning in part of the Pentagon. No death figures have been released yet.

5:20 p.m.: The 47-story Building 7 of the World Trade Center complex collapses. The evacuated building is damaged when the twin towers across the street collapse earlier in the day. Other nearby buildings in the area remain ablaze.

5:30 p.m.: CNN Senior White House Correspondent John King reports that U.S. officials say the plane that crashed in Pennsylvania could have been headed for one of three possible targets: Camp David, the White House or the U.S. Capitol building.

6 p.m.: Explosions are heard in Kabul, Afghanistan, hours after terrorist attacks targeted financial and military centers in the United States. The attacks occurred at 2:30 a.m. local time. Afghanistan is believed to be where bin Laden, who U.S. officials say is possibly behind Tuesday's deadly attacks, is located. U.S. officials say later that the United States had no involvement in the incident whatsoever. The attack is credited to the Northern Alliance, a group fighting the Taliban in the country's ongoing civil war.

6:10 p.m.:Giuliani urges New Yorkers to stay home Wednesday if they can.

6:40 p.m.: Rumsfeld, the U.S. defense secretary, holds a news conference in the Pentagon, noting the building is operational. "It will be in business tomorrow," he says.

6:54 p.m.: Bush arrives back at the White House aboard Marine One and is scheduled to address the nation at 8:30 p.m. The president earlier landed at Andrews Air Force Base in Maryland with a three-fighter jet escort. CNN's King reports Laura Bush arrived earlier by motorcade from a "secure location."

7:17 p.m.: U.S. Attorney General John Ashcroft says the FBI is setting up a Web site for tips on the attacks: www.ifccfbi.gov. He also says family and friends of possible victims can leave contact information at 800-331-0075.

7:02 p.m.: CNN's Paula Zahn reports the Marriott Hotel near the World Trade Center is on the verge of collapse and says some New York bridges are now open to outbound traffic.

7:45 p.m.: The New York Police Department says that at least 78 officers are missing. The city also says that as many as half of the first 400 firefighters on the scene were killed.

8:30 p.m.: President Bush addresses the nation, saying "thousands of lives were suddenly ended by evil" and asks for prayers for the families and friends of Tuesday's victims. "These acts shattered steel, but they cannot dent the steel of American resolve," he says. The president says the U.S. government will make no distinction between the terrorists who committed the acts and those who harbor them. He adds that government offices in Washington are reopening for essential personnel Tuesday night and for all workers Wednesday.

9:22 p.m.: CNN's McIntyre reports the fire at the Pentagon is still burning and is considered contained but not under control.

9:57 p.m.: Giuliani says New York City schools will be closed Wednesday and no more volunteers are needed for Tuesday evening's rescue efforts. He says there is hope that there are still people alive in rubble. He also says that power is out on the westside of Manhattan and that health department tests show there are no airborne chemical agents about which to worry.

10:49 p.m.: CNN Congressional Correspondent Jonathan Karl reports that Attorney General Ashcroft told members of Congress that there were three to five hijackers on each plane armed only with knives.

10:56 p.m: CNN's Zahn reports that New York City police believe there are people alive in buildings near the World Trade Center.