American Airlines Flight 11 and United Airlines Flight 175, both Boston to LA flights, were taken over by 5 hijackers each and crashed into the Twin Towers of the World Trade Center.

Times - Deaths
Airline WTC
Collapse
Crash
floors
Deaths
Takeoff Hijacking Crash Passengers
& crew
Building
& ground
North Tower 7:57 a.m. 8:19 a.m. 8:46 a.m. 9:59 a.m. 93-99 87 1,470
South Tower 8:14 a.m. 8:45 a.m. 9:03 a.m. 10:28 a.m. 77-85 60 695
Emergency workers 441
Total 2,753 (based on names engraved on the memorial)

Estimates were 5,000 to 7,000 people were in each tower when the attack began, which would mean about 18% perished. The towers had 110 stories.

In the South Tower one stairwell remained intact, allowing 18 people to escape from above the point of impact. In the South Tower 630 people died, fewer than half the number killed in the North Tower. Casualties in the South Tower were significantly reduced by the decision of some occupants to start evacuating when the North Tower was struck.

At least 200 people fell or jumped to their deaths.

Debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures. 7 World Trade Center collapses at 5:20.

Timelines: Summary of timeline at Wikipedia. See it for flight 77 and 93 information.
8:14: Flight 11 fails to heed air traffic controller's instruction to climb to 35,000 feet.
8:19: Betty Ong, a flight attendant on Flight 11 alerts American Airlines via an airphone of stabbings and loss of communication to cockpit.
8:20: The Federal Aviation Administration's Boston Center flight controllers decide that Flight 11 has probably been hijacked.
8:25: Boston Center flight controllers alert other flight control centers regarding Flight 11. NORAD (North American Aerospace Defense Command) is not yet alerted.
8:34: Dan Bueno from Boston Center notifies the tower controller at Otis Air National Guard Base at Cape Cod of the hijacking of Flight 11. The controller directs Bueno to contact NEADS (Northeast Air Defense Sector), the northeast sector of NORAD. Two F15 pilots begin to suit up.
8:37:52: Boston Center control notifies NEADS of the hijacking of Flight 11, the first notification received by NORAD that American 11 had been hijacked. The controller requests military help to intercept the jetliner.
8:46: Two F-15 fighter jets are ordered to scramble from Otis Air National Guard Base in Massachusetts, intended to intercept Flight 11. Because Flight 11's transponder is off, the pilots do not know the location of their target.
8:46:30 Flight 11 crashes at roughly 466 mph into the north face of the North Tower.
8:52: on Flight 175 a flight attendant aboard calls a United Airlines office in Chicago, reporting that the flight had been hijacked, both pilots had been killed, a flight attendant had been stabbed, and the hijackers were probably flying the plane.
8:52: The F-15s at Otis Air National Guard Base are airborne. Still lacking an intercept vector to Flight 11 (and not aware that it has already crashed), they are sent to military controlled airspace off Long Island and ordered to remain in a holding pattern until between 9:09 and 9:13.
8:55 (approx.): Announcements are made over the building-wide PA system by officials in the still-undamaged South Tower of the World Trade Center, reporting that the building is "secure" and that people should return to their offices.
9:01 to 9:02: A manager from the FAA's (Federal Aviation Agency) New York Center tells the Air Traffic Control System Command Center in Herndon, Virginia, "We have several situations going on here. It's escalating big, big time. We need to get the military involved with us.
9:02: Evacuation is ordered by FDNY Battalion Chief Joseph Pfeifer (North Tower). An announcement is made over the PA system to evacuate the buildings.
9:03:02: Flight 175 crashes at about 590 mph into the south face of the South Tower.
A massive evacuation begins in the South Tower below its impact zone. One of the stairwells in the South Tower remains unblocked from the top to the bottom of the tower, but is filled with smoke. This leads many people to mistakenly go upwards towards the roof for a rooftop rescue that never comes. The Port Authority kept the two sets of heavy metal doors leading to the building's only roof exit tightly locked.
9:06: The FAA bans takeoffs of all flights bound to or through the airspace of New York Center from airports in that Center and the three adjacent Centers -- Boston, Cleveland, and Washington.
9:06: The FAA bans takeoffs of all flights bound to or through the airspace of New York Center from airports in that Center and the three adjacent Centers -- Boston, Cleveland, and Washington.
9:11: The last PATH train leaves the World Trade Center. The station is vacant when the towers collapse.
9:21: All bridges and tunnels into Manhattan closed. At 9:40 a.m., the FAA grounded all aircraft within the continental U.S., and aircraft already in flight were told to land immediately. All international civilian aircraft were either turned back or redirected to airports in Canada or Mexico, and all international flights were banned from landing on U.S. soil for three days.
9:43: The White House and the Capitol are evacuated and closed.
10:20: President Bush, aboard Air Force One, tells Vice President Cheney that he has authorized a shootdown of aircraft if necessary.
10:24: Two men who were being evacuated through the underground shopping mall below the South Tower when it collapsed on them are able to climb up through thirty feet of debris to safety.
12:01 (approximately): Fourteen people, including twelve firefighters, who were in a section of a stairwell in the North Tower that held together during the collapse, climb the stairs to the top of the Ground Zero rubble field.
3:00 (approx.): Pasquale Buzzelli, who lost consciousness in a North Tower stairway during the collapse, awakens to find himself lying atop the debris with only a fractured foot.
5:20:33: 7 World Trade Center, also known as the Salomon Bros. Building, a 47-story building collapses.
8:00 (approx.): Port Authority Police Officer Will Jimeno, who was in an underground corridor between the two towers, is found alive in the rubble, and eventually freed at approximately 11:00 p.m.
10:00: There are reports (later proven incorrect) of many survivors buried in rubble in New York making cell phone calls. Only two more survivors will be pulled from the rubble on September 12 and neither of them had made cell phone calls.

In 2011 one of the Air National Guard F-16 pilots revealed they were scrambled without any weapons and were prepared to ram flight 93, hopefully ejecting just before impact.
She also revealed her father was a United pilot and could have been flying flight 93 (he wasn't).
See Bangor Daily News

Links:
September 11 News.com - News Archives - Images, & Newspapers from the Attacks at the WTC & Pentagon.
USATODAY.com - For many on Sept. 11, survival was no accident
9/11 "world trade center" pictures - Google Search
September 11 attacks - Wikipedia, the free encyclopedia
Timeline for the day of the September 11 attacks - Wikipedia
September 11 2001 Video. - YouTube
The 9/11 World Trade Center attacks, through the eyes of a former NYC fire official | NJ.com
The World Trade Center Attack: The Official Documents
9/11 Conspiracy Theories - Debunking the Myths - World Trade Center - Popular Mechanics

9/11 Memorial
BOATLIFT, An Untold Tale of 9/11 Resilience


last updated 17 Sept 2011