On September 11, 2001, terrorists allegedly associated with al Qaeda hijacked four US commercial airliners, two of which were crashed into the World Trade Center Towers in New York City, with a third hitting the Pentagon in Washington DC. The fourth plane went down in a field near Shanksville, Pennsylvania. The attacks spawned an immediate tightening of aviation security regulations and in October 2001 led to Congressional passage of the controversial USA PATRIOT Act, giving the executive broad new national security powers.