Shortly before 8 a.m.: American Airlines Flight 11 leaves Boston for Los Angeles.

8:50 a.m.: Plane hits North Tower of World Trade Center. Clyde Ebanks, vice president of an insurance company, was at a meeting on the 103rd floor of the South Tower of the World Trade Center when his boss said, "Look at that!"

9 a.m.: Luigi Ribaudo -- who works nearby, in Tribeca -- heard a twin-engine plane making what he said was a strange noise. He looked up; he saw a plane that was "too low." "It was going to hit something and it hit and exploded inside," he said. It was American Flight 77, a Boeing 757, operating from Washington Dulles to Los Angeles.

9:30 a.m.: An aircraft crashed at the Pentagon in Washington. The nerve center of the nation's military burst into flames and a portion of one side of the five-sided structure collapsed, sending billows of smoke over the capital.

9:45 a.m. ET: The White House's West Wing is evacuated when Secret Service agents learned that it might be a target of the terrorist attacks. Soon thousands of government employees were leaving their offices all over the city. Also, Federal Aviation Administration had grounded air traffic nationwide.

9:50 a.m.: One World Trade Center collapsed.

Around 10 a.m.: A United Airlines flight from Newark to San Francisco crashed 80 miles southeast of Pittsburgh. Minutes earlier, a man who said he was a passenger on the plane told an emergency dispatcher in a cell phone call: "We are being hijacked, we are being hijacked!"

At exactly 10:30 a.m.: The second tower of the World Trade Center collapsed. The top of the building exploded with smoke and dust. There were no flames, just an explosion of debris and dust and smoke, and then more huge vast clouds swept down to the streets. People were knocked to the ground onto their faces as they were running from the building toward cover. And then the same huge clouds of smoke, dust and debris and came through the buildings and blocked out the sun.

Noon: United Airlines announced that another of its planes had gone down. No location was given; it was not confirmed whether this was the plane that hit the Pentagon.