Donald Rumsfeld
The United States was attacked on September 11, 2001 by al Qaeda terrorists who hijacked commercial airliners and crashed them in coordinated strikes into both towers of the World Trade Center in Lower Manhattan, New York City and the Pentagon in Washington, D.C. The fourth plane crashed into a field in Shanksville, Pennsylvania, and its target was likely a prominent building in Washington, D.C., most probably either the Capitol Building or the White House. Within three hours of the start of the first hijacking and two hours after American Airlines Flight 11 struck the World Trade Center, Rumsfeld raised the defense condition signaling of the United States offensive readiness to DEFCON 3, the highest it had been since the Arab-Israeli war in 1973.
Al-Qaeda
In December 1998, the Director of the CIA Counterterrorism Center reported to President Bill Clinton that al-Qaeda was preparing to launch attacks in the United States, and that the group was training personnel to hijack aircraft. On September 11, 2001, al-Qaeda attacked the United States, hijacking four airliners within the country and deliberately crashing two into the twin towers of the World Trade Center in New York City. The third plane crashed into the western side of the Pentagon in Arlington County, Virginia. The fourth plane was crashed into a field in Shanksville, Pennsylvania. In total, the attackers killed 2,977 victims and injured more than 6,000 others.