This Day in History:, 9/11 Terrorist Attacks. September 11, 2001. Masterminded by Saudi national Osama bin Laden, 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. . Hours later, a deeply shocked nation watched the collapse of the Twin Towers live on national television. . A third plane was flown into the west side of the Pentagon military headquarters. . Passengers on the fourth plane, United Flight 93, overtook the terrorists. The plane crashed into a Shanksville, PA, field rather than its unknown target. . Nearly 3,000 people died in the attacks, the most significant in American history. . Nearly 10,000 people who were exposed to the debris of the fallen towers have been diagnosed with 9/11-related cancer. . The economic toll of the attacks has approached $1 trillion. . In response to 9/11, the Department of Homeland Security was created. . On the ten-year anniversary of the attack, the World Trade Center memorial "Reflecting Absence" was opened to the public