The 19 hijackers pass through airport security checkpoints to board their flights at Boston Logan, Newark, and Washington Dulles airports.

At Logan International Airport in Boston, three of the five hijackers from American Airlines Flight 11 are selected by CAPPS (Computer Assisted Passenger Prescreening System) which identifies passengers subject to special security measures. Their luggage is held off the plane briefly but they are permitted to board.

United Flight 175 also boards at Logan Airport, but no hijackers are stopped or searched.

At Washington Dulles Airport, three of the hijackers are also flagged by CAPPS before boarding American Airlines Flight 77 but are not stopped.

Before boarding United Flight 93 at Newark Liberty International Airport, only Ahmad al Haznawi is selected by CAPPS. His bag is screened for explosives but he is not stopped.

Before checking into Flight 11, hijacker Mohammed Atta takes a call from Marwan al Shehhi, who is at Terminal C of Logan airport on his way to check into Flight 175. It is the last conversation these two will ever have.

American Airlines Flight 11 departs Logan International Airport en route to Los Angeles International Airport. The plane is carrying 81 passengers, two pilots and nine flight attendants.

United Airlines Flight 175 departs 14 minutes late from Logan International Airport en route to Los Angeles International Airport. The plane is carrying 56 passengers, two pilots and seven flight attendants.

American Airlines Flight 11 is hijacked and diverted to New York City.

A flight attendant on Flight 11 alerts American Airlines of a hijacking in progress.

American Airlines Flight 77 departs 10 minutes late from Washington Dulles International Airport en route to Los Angeles International Airport. The plane is carrying 58 passengers, two pilots and four flight attendants.

American Airlines Flight 11's transponder signal is turned off but the plane remains on radar screens.

In accordance with federal aviation protocol, Boston Center traffic control managers started notifying their chain of command that American 11 had been hijacked.

The Northeast American Defense Sector (NEADS) of the North American Aerospace Defense Command (NORAD) is notified of the hijacking of American Airlines Flight 11 by the U.S. Federal Aviation Administration's (FAA) Boston Center Control. The controller requests military help to intercept the aircraft.

United Airlines Flight 93 departs 42 minutes late due to runway congestion from Newark International Airport en route to San Francisco International Airport. The plane is carrying 37 passengers, two pilots and five flight attendants.

United 175 flight crew complete their report on a "suspicious transmission" overheard from another plane (which turned out to have been Flight 11) just after takeoff. This was United 175's last communication with the ground.

United Airlines Flight 175 is taken over by five hijackers who use knives, Mace and the threat of a bomb. Both pilots are killed and the plane is diverted southward to New York City.

F-15 fighter jets are dispatched from Otis Air Force Base in Mass., but because Flight 11's transponder is off, Air Force pilots do not know which direction to travel to meet the plane. NEADS personnel spend the next several minutes watching their radar scopes waiting for Flight 11 to reappear.

American Airlines Flight 11 crashes at a speed of roughly 470 mph into the north side of the North Tower of the World Trade Center, between floors 93 and 99. The impact of the crash incinerates everything in its way, including wallboards, filing cabinets and computers.

FAA air traffic control receives its first indication that something was abnormal on Flight 175: its beacon codes change twice within one minute.

CNN breaks into a commercial and is the first news agency to report the explosion at the World Trade Center. Anchor Carol Lin reports, "Yeah. This just in. You're looking at obviously a very disturbing live shot there. That is the World Trade Center, and we have unconfirmed reports this morning that a plane has crashed into one of the towers of the World Trade Center. CNN Center is just beginning to work on this story, obviously, calling our sources and trying to figure out exactly what happened, but clearly something relatively devastating happening this morning there on the south end of the island of Manhattan. That is once again, a picture of one of the towers of the World Trade Center."

An announcement over the public-address system in the South Tower states that the incident has occurred in the other building and advises tenants, generally, that their building is safe, that they should remain on or return to their offices or floors. Some don't hear the announcement, others ignore it and evacuate anyway, while others congregate in common areas like the 78th floor sky lobby.

Unaware that their intended target has already crashed into the World Trade Center, the F-15s from Otis Air Force Base are sent to military-controlled airspace off Long Island and ordered to remain in a holding pattern until between 9:09 and 9:13 a.m.

American Airlines Flight 77 is hijacked by five terrorists using knives and box cutters to enter the cockpit. The plane is diverted toward Washington, D.C.

As part of a scheduled visit to promote his education policy, President George W. Bush enters a classroom at Emma E. Booker Elementary School in Sarasota, Fla. Bush is aware that a commercial airline has struck the North Tower of the World Trade Center, but continues his scheduled visit.

NORAD NEADS is notified by the FAA's New York Center of the hijacking of Flight 175.

United Airlines Flight 175 crashes at a speed of about 590 mph into the south side of the South Tower of the World Trade Center, crashing through the 77th and 85th floors. By this time, several media organizations are covering the first plane crash and millions see the impact live.

The FAA's Boston Air Route Traffic Control Center stops all departures from airports in its jurisdiction in the New England Region. All New York-area airports are also shut down.

As President Bush begins to read with the students at Booker Elementary, Chief of Staff Andrew Card interrupts to whisper to him, "A second plane hit the second tower, America is under attack." President Bush continues the lesson for another five to seven minutes, rather than alarm the students.

The FAA bans all takeoffs nationwide for flights going to or through New York City. New York City airspace is closed.

The Otis Air Force-based F-15 fighters leave military airspace near Long Island, bound for Manhattan.

President Bush leaves the classroom at Booker Elementary School and is brought to a secure room where he is briefed by staff and watches television coverage of the tragedy unfolding in New York City.

United Airlines Flight 93 receives a warning from flight dispatch, "Beware any cockpit intrusion - two aircraft hit World Trade Center."

The Otis Air Force-based F-15s close the airspace over Manhattan.

The FAA grounds all domestic flights.

The United Airlines Flight 93 pilot asks for confirmation of the 9:24 a.m. message, "Beware any cockpit intrusion - two aircraft hit World Trade Center."

United Airlines Flight 93 is hijacked.

President Bush makes his first public statements about the tragedy to teachers and students at the elementary school. He tells them "we've had a national tragedy" and that he will be returning to Washington, D.C. immediately. He then leads a moment of silence.

A man on United Airlines Flight 93 transmits a message to what he believes to be only the passengers, but is overheard by air traffic control: "Ladies and gentlemen, here the captain, please sit down, keep remaining sitting. We have a bomb on board. So sit."

American Airlines Flight 77 crashes into the southwestern side of the Pentagon in Arlington, Va. All on board are killed, as are 125 Pentagon personnel.

Air traffic controllers overhear the final transmission from the United Airlines Flight 93 hijackers: "Uh, this is the captain. Would like you to all remain seated. There is a bomb on board, and are going back to airport, and to have our demands…please remain quiet."

The White House and the Capitol are evacuated.

After burning for approximately 56 minutes from the impact of Flight 175, the South Tower of the World Trade Center collapses, covering lower Manhattan in a gray-white cloud of dust and debris.

United Airlines Flight 93 crashes into a coal-mining area southeast of Pittsburgh in Somerset County, Pa.

A large section of the west side of the Pentagon collapses.

As if being split down the center, the North Tower of the World Trade Center collapses from the top down. This second collapse is also viewed live on television and heard on radio.

President Bush makes a brief statement from Barksdale Air Force Base in Shreveport, La., saying that "freedom itself was attacked this morning by a faceless coward and freedom will be defended." He adds, "The United States will hunt down and punish those responsible for these cowardly acts."

The Taliban government of Afghanistan denounces the attacks.

Airspace over the continental United States is closed except for military and emergency flights.

Washington, D.C., Mayor Anthony A. Williams declares a state of emergency and the National Guard is deployed onto the streets of Washington.

President Bush leaves Barksdale Air Force Base aboard Air Force One and flies to an Air Force base in Nebraska.

The FAA announces there will be no U.S. commercial air traffic until noon EDT on Wednesday, September 12, 2001, at the earliest.

National news outlets report that senior officials in the U.S. federal intelligence community believe Osama bin Laden to be the number one suspect behind the attacks.

The American Stock Exchange, NASDAQ and the New York Stock Exchange announce they will remain closed Wednesday, September 12.

After being damaged by the collapse of the North and South Towers, the evacuated 47-story Building 7 of the World Trade Center complex collapses. Other nearby buildings continue to burn.

President Bush arrives back at the White House.

President Bush addresses the nation, saying, "thousands of lives were suddenly ended by evil" and asks for prayers for the families and friends of the 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 have reopened for essential personnel Tuesday night and for all workers Wednesday.