5:45 a.m. - Hijackers pass through security in Portland, Maine, and board a flight to Boston, where they connect to American Airlines Flight 11.
6:00 a.m. - Polling stations open, as September 11 was a primary election day in New York City.
7:59 a.m. - Flight 11 takes off from Boston for Los Angeles. Eleven crew members, 76 passengers, and five hijackers are on board.
8:15 a.m. - Flight 175 takes off from Boston for Los Angeles. Nine crew members, 51 passengers, and five hijackers are on board.
8:19 a.m. - Flight 11 crew members alert ground personnel that a hijacking is underway. Shortly before, one hijacker stabbed a passenger, Daniel Lewin, who was seated in front of him. Officials speculate that Lewin may have tried to stop the hijackers and was likely the first victim of the attacks.
MORE: 9/11 attacks by the numbers
8:20 a.m. - American Airlines Flight 77, en route to Los Angeles, takes off from Washington, D.C. Six crew members, 53 passengers, and five hijackers are on board.
8:24 a.m. - Attempting to communicate with passengers, a hijacker contacts air traffic control, unwittingly alerting controllers to the attacks.
8:37 a.m. - Boston air traffic control alerts the military. Air National Guard jets in Massachusetts are mobilized to follow Flight 11.
8:42 a.m. San Francisco-bound United Airlines Flight 93 takes off at Newark following a delay. Seven crew members, 33 passengers, and four hijackers are on board.
8:46 a.m. - Flight 11 crashes into floors 93 through 99 of the North Tower.
8:50 a.m. - President George W. Bush is alerted. His advisors assume this is a tragic accident.
8:55 a.m. - The South Tower is declared secure.
8:59 a.m. - Port Authority police order the evacuation of both towers. A minute later, the order is expanded to the entire World Trade Center complex.
9:00 a.m. - A flight attendant aboard Flight 175 alerts air traffic control that a hijacking is underway.
9:03 a.m. - Flight 175 crashes into floors 77 through 85 of the South Tower.
9:05 a.m. - President Bush learns that a second plane has crashed into the World Trade Center. Twenty-five minutes later, he addresses Americans, saying that "terrorism against our nation will not stand."
9:05 a.m. - Flight 77 passenger Barbara Olson calls her husband, U.S. Solicitor General Theodore Olson, who alerts other federal officials of the hijacking.
9:36 a.m. - Secret Service agents evacuate Vice President Dick Cheney to the Presidential Emergency Operations Center beneath the White House.
9:37 a.m. - American Airlines Flight 77 crashes into the Pentagon. The crash and fire kill 59 on the plane and 125 on the ground.
9:42 a.m. - The FAA grounds all flights.
9:45 a.m. - The White House and U.S. Capitol are evacuated.
9:59 a.m. - The South Tower collapses in 10 seconds after burning for 56 minutes. More than 800 people in and around the building are killed.
10:03 a.m. - United Airlines Flight 93 crashes near Shanksville, Pennsylvania after passengers and crew storm the cockpit. Forty passengers and crew on board perish.
10:15 a.m. - The Pentagon's outer ring collapses.
10:28 a.m. - The North Tower collapses after burning for 102 minutes. More than 1,600 in and around the building are killed.
11:02 a.m. - New York City Mayor Rudy Giuliani orders the evacuation of Lower Manhattan.
12:16 p.m. - The last flight still in the air above the continental United States lands.
12:30 p.m. - A group of 14 survivors emerge from a North Tower stairwell.
3:00 p.m. - A survivor, Pasquale Buzzelli, is rescued from the rubble of the North Tower.
5:20 p.m. - After burning for hours, 7 World Trade Center collapses, There are no casualties.
8:30 p.m. - President Bush addresses the nation from the White House, assuring Americans that a search is underway for "those who are behind these evil acts."
10:30 p.m. - Rescuers locate and extract two PAPD officers injured but alive in debris of the World Trade Center.