In 1996, Osama bin Laden, a former Saudi Arabian citizen and leader of al-Qaeda, issued his first fatwa, which declared jihad against the United States and demanded the expulsion of American troops from Arabia. Bin Laden interpreted Muhammad as banning non-Muslims from Arabia and considered the U.S. troop presence a provocation. Prior to 1996, bin Laden had lived in Sudan, but was exiled after Saudi and U.S. pressure led the Sudanese government to ask him to leave. He then returned to Afghanistan, where the Taliban allowed al-Qaeda to establish a base of operations.
In 1996, Khalid Sheikh Mohammed presented bin Laden with a plan for attacks, which was initially deemed too elaborate and rejected. The plan was revisited in late 1998 or early 1999. Bin Laden provided leadership and financial support. Mohammed had previously been an organizer and financier of al-Qaeda’s 1993 bombing of the World Trade Center, and was the uncle of Ramzi Yousef, the lead bomber in that attack.
On the morning of September 11, 2001, nineteen hijackers took control of four commercial airliners. Flight 11 struck the North Tower, followed by Flight 175 into the South Tower. Flight 77 hit the Pentagon, and Flight 93 crashed in Pennsylvania. The Federal Aviation Administration ordered a ground stop for all traffic in U.S. airspace, requiring all airborne aircraft to return to their point of origin or divert to Canada. The actions undertaken in Canada to support incoming aircraft and their occupants were collectively titled Operation Yellow Ribbon. The crashes of Flight 11 and Flight 175 were the deadliest aviation disasters in history. Both towers of the World Trade Center collapsed within an hour and forty-two minutes, destroying the remaining five structures in the complex.
Following the attacks, the United States launched the global war on terror, seeking to eliminate hostile groups deemed terrorist organizations and the governments purported to support them. The U.S. invaded Afghanistan, seeking to eliminate al-Qaeda and governments supporting them.
The attacks killed 2,977 people, injured thousands more, and caused at least US$10 billion in infrastructure and property damage. The attacks remain the deadliest terrorist attack in history, and were the deadliest incidents for firefighters and law enforcement personnel in American history, killing 343 and 72 members, respectively.
The New York City Fire Department (FDNY) deployed more than 200 units to the World Trade Center. Their efforts were supplemented by off-duty firefighters and emergency medical technicians. The FDNY, the NYPD, and the PAPD did not coordinate efforts and performed redundant searches for civilians.
The 9/11 attacks resulted in immediate responses, including domestic reactions, closings and cancellations, hate crimes, international responses, and military responses. Shortly after the attacks, the September 11th Victim Compensation Fund was created to compensate the victims of the attacks and their families.
Immediately after the attacks, the Federal Bureau of Investigation (FBI) started PENTTBOM, the largest criminal inquiry in U.S. history. At its height, more than half of the FBI's agents worked on the investigation. The FBI concluded that there was "clear and irrefutable" evidence linking al-Qaeda and bin Laden to the attacks.
Within hours of the attack, a search and rescue operation was launched. After months of around-the-clock operations, the World Trade Center site was cleared by the end of May 2002. The damaged section of the Pentagon was rebuilt and occupied within a year of the attacks. The temporary World Trade Center PATH station opened in late 2003. Construction of the new 7 World Trade Center was completed in 2006. The construction of One World Trade Center began in April 2006 and reached its full height in May 2013. One WTC finished construction and opened on November 3, 2014.