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.
September 11 attacks
Immediately after the attacks, the Federal Bureau of Investigation started PENTTBOM, the largest criminal inquiry in the history of the United States. At its height, more than half of the FBI's agents worked on the investigation and followed a half-million leads. The FBI concluded that there was "clear and irrefutable" evidence linking al-Qaeda and bin Laden to the attacks.
The FBI was quickly able to identify the hijackers, including leader Mohamed Atta, when his luggage was discovered at Boston's Logan Airport. Atta had been forced to check two of his three bags due to space limitations on the 19-seat commuter flight he took to Boston. Due to a new policy instituted to prevent flight delays, the luggage failed to make it aboard American Airlines Flight 11 as planned. The luggage contained the hijackers' names, assignments, and al-Qaeda connections. "It had all these Arab-language papers that amounted to the Rosetta stone of the investigation", said one FBI agent. Within hours of the attacks, the FBI released the names and in many cases the personal details of the suspected pilots and hijackers. On September 27, 2001, they released photos of all 19 hijackers, along with information about possible nationalities and aliases. Fifteen of the men were from Saudi Arabia, two from the United Arab Emirates, one from Egypt, and one from Lebanon.