LIVE ABC News Special Report: 9/11 Twenty Years Later: America Remembers

Here's a timeline of September 11, 2001, provided by ABC News.

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.

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 people on board, excluding the hijackers, 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.

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