September 11, 2001 marks one of the most tragic and deadliest days in American history. Four American airplanes were hijacked by 19 members of the terrorist group, al Qaeda. The terrorists flew two of the planes into the Twin Towers of the World Trade Center in New York City. Within two hours of the attack, the world watched the collapse of the Twin Towers live on television. The third plane was flown into the west side of the Pentagon military headquarters. Passengers on the fourth plane, United Flight 93, overtook some of the hijackers before the plane crashed into a field in Shanksville, Pennsylvania, rather than its unknown target. Nearly 3,000 people died in the attacks. Nearly 10,000 people who were exposed to the debris of the fallen towers have been diagnosed with 9/11-related illnesses. The economic toll of the attacks has approached $1 trillion. In response to 9/11, the U.S. Department of Homeland Security was created. On the 10-year anniversary of the attack, the World Trade Center memorial "Reflecting Absence" was opened to the public.