World Trade Center Health Program
On September 11, 2001, 19 terrorists associated with the Islamist militant group al-Qaeda hijacked four passenger jets in a series of suicide attacks on the United States, killing 2,996 people and injuring more than 6,000 others. Hijackers flew planes into two main towers of the World Trade Center, resulting in the collapse of three buildings of the World Trade Center complex, and the Pentagon, resulting in the partial destruction of the western side in the building. The resulting dust cloud due to the collapse of the World Trade Center covered Manhattan for days and contained thousands of tons of toxic debris, including asbestos and other known carcinogens. Another hijacked plane crashed into a field in Shanksville, Pennsylvania following an attempt by the passengers to retake control of the plane. No one in the airliners survived.
As of 2011, approximately 18,000 people have received medical treatment for illnesses related to toxic dust from the World Trade Center site. A study of rescue workers released in April 2010 found that all those studied had impaired lung functions, and that 30–40% were reporting little or no improvement in persistent symptoms that started within the first year of the attack. In 2011 a major research study showed significant long term medical and psychological effects among first responders to the World Trade Center site. These effects include elevated levels of asthma, sinusitis, Gastroesophageal reflux disease and posttraumatic stress disorder.
September 11 attacks advance-knowledge conspiracy theories
There have also been allegations that the hijackers' preparations may have been given assistance by U.S. intelligence. According to CBS News, "two of the Sept. 11 hijackers who lived in San Diego in 2000 rented a room from a man who reportedly worked as an undercover FBI informant... the FBI informant prayed with them and even helped one open a bank account." Doubts have been raised about the speed with which the hijackers were identified, leading to suggestions that the FBI already had the names of the hijackers in advance. In his book "Against All Enemies", Richard Clarke said that at 9:59 am on September 11, which is the time when WTC2 collapsed and 8 minutes before NORAD even knew Flight 93 had been hijacked, the FBI already had a list of the 19 alleged hijackers. A former high-level intelligence official said that "Whatever trail was left was left deliberately—for the F.B.I. to chase."