(from Wikipedia article: http://en.wikipedia.org/wiki/September_11_attacks)
Early on the morning of September 11, 2001, nineteen hijackers took control of four commercial airliners en route to San Francisco and Los Angeles from Boston, Newark, and Washington, D.C. Planes with long flights were intentionally selected for hijacking because they would be heavily fueled. At 8:46 a.m., American Airlines Flight 11 crashed into the World Trade Center's North Tower, followed by United Airlines Flight 175, which hit the South Tower at 9:03 a.m. Another group of hijackers flew American Airlines Flight 77 into the Pentagon at 9:37 a.m.
A fourth flight, United Airlines Flight 93, crashed near Shanksville, Pennsylvania at 10:03 a.m. local time after the passengers on board engaged in a fight with the hijackers. Its ultimate target is believed to have been either the Capitol or the White House. Flight 93's cockpit voice recorder revealed that crew and passengers attempted to seize control of the plane from the hijackers after learning through phone calls that similarly hijacked planes had been crashed into buildings that morning One of the hijackers gave the order to roll the plane once it became evident that they would lose control of the plane to the passengers. Soon afterward, the aircraft crashed into a field near Shanksville.
Some passengers were able to make phone calls using the cabin airphone service and mobile phones, and provide details: the presence of several hijackers aboard each plane; that mace or other noxious chemical spray, such as tear gas or pepper spray was used; that some people aboard had been stabbed. Reports indicated that during two of the flights, the hijackers stabbed and killed aircraft pilots, flight attendants and in at least one case, a passenger. The 9/11 Commission established that two of the hijackers had recently purchased Leatherman multi-function hand tools. Others had purchased an assortment of knives and other types of blades.
A flight attendant on Flight 11, a passenger on Flight 175, and passengers on Flight 93 mentioned that the hijackers had bombs, but one of the passengers also said he thought the bombs were fake. The FBI found no traces of explosives at the crash sites, and the 9/11 Commission concluded the bombs were probably fake.
Three buildings in the World Trade Center Complex collapsed due to structural failure The south tower (2 WTC) fell at approximately 9:59 a.m., after burning for 56 minutes in a fire caused by the impact of United Airlines Flight 175. The north tower (1 WTC) collapsed at 10:28 a.m., after burning for approximately 102 minutes When the north tower collapsed, debris fell on the nearby 7 World Trade Center building (7 WTC) damaging it and starting fires. These fires burned for hours and compromised the building's structural integrity, which led to the crumbling of the east penthouse at 5:20 p.m. and complete collapse at 5:21 p.m.
All aircraft within the continental U.S. were grounded, 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. The attacks created widespread confusion among news organizations and air traffic controllers across the United States. Among the unconfirmed and often contradictory reports aired throughout the day by news sources, one of the most prevalent was the report that a car bomb had been detonated at the U.S. State Department's headquarters in Washington, D.C. Another jet—Flight 1989—was suspected of having been hijacked, but this too turned out to be false after it responded to controllers and landed safely in Cleveland, Ohio.
In a September 2002 interview, Khalid Sheikh Mohammed and Ramzi bin al-Shibh, who are believed to have organized the attacks, said the fourth hijacked plane was heading for the United States Capitol, not for the White House. Atta thought the White House might be too tough a target and sought an assessment from fellow pilot Hani Hanjour. They also said al-Qaeda initially planned to target nuclear installations rather than the World Trade Center and the Pentagon, but decided against it, fearing it could "get out of control". Final decisions on targeting, according to Mohammed, was left in the hands of the pilots.