Nearly 3,000 people were killed when al-Qaida hijackers crashed four jetliners into the twin towers, the Pentagon and a field in southwest Pennsylvania on Sept. 11, 2001.
Here is an outline of the primary events that transpired, starting with the first plane hitting the North Tower, according to the 9/11 Memorial & Museum.
8:46 a.m.
Five hijackers crash American Airlines Flight 11 into floors 93 through 99 of the North Tower. The 76 passengers and 11 crew members on board and hundreds inside the building die instantly. Hundreds of people are trapped above the 91st floor because the crash severs all three emergency stairwells.
8:50 a.m.
Then President George W. Bush is informed of the plane crash as he sits in a Florida elementary school classroom. He and his advisers assume it is an accident.
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8:59 a.m.
The Port Authority orders the evacuation of both towers. A minute later, they evacuate the entire World Trade Center complex.
9:03 a.m.
Five hijackers crash United Airlines Flight 175 into floors 77 through 85 of the South Tower, killing the 51 passengers and nine crew members onboard the aircraft and an unknown number of people inside the building. Many were trapped in elevators or on floors above the impact zone. People are seen jumping to their deaths from the towers.
At this point, the estimated death count is 50 to more than 200.
The NYPD calls for a second Level 4 mobilization.
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9:05 a.m.
President Bush learns the second plane has crashed. He delivers 1-minute remarks to the public from Florida before leaving. At this point, it's a terror investigation.
9:37 a.m.
Five hijackers crash American Airlines Flight 77 into the Pentagon. The 53 passengers and six crew members on board die. Another 100-plus military and civilian personnel on the ground are also killed.
9:59 a.m.
The South Tower collapses in 10 seconds after burning for nearly an hour. More than 800 people inside the building at the time are killed.
10:03 a.m.
Four hijackers crash Flight 93 in a field near the town of Shanksville in Pennsylvania, about 20 minutes flying time from Washington, D.C., after passengers and crew storm the cockpit. The 33 passengers and seven crew on board die.
10:15 a.m.
Part of the Pentagon collapses.
10:37 a.m.
The North Tower collapses. More than 1,600 people are killed as a result of the attack on that building.
11:02 a.m.
Then-New York City Mayor Rudy Giuliani calls for the evacuation of Lower Manhattan.
Early afternoon
Rescue efforts continue at the World Trade Center site, which is now being referred to as ground zero.
5:20 p.m.
7 World Trade Center collapses after burning for hours. It had been evacuated well prior.
8:30 p.m.
Bush addresses the nation from the White House.
“The search is underway for those who are behind these evil acts," he says. "I’ve directed the full resources of our intelligence and law enforcement communities to find those responsible and to bring them to justice. We will make no distinction between the terrorists who committed these acts and those who harbor them."
