PENTTBOM (sometimes referred to as PENTTBOMB[1]) is the codename for the Federal Bureau of Investigation's probe into the September 11 attacks of 2001, the largest criminal inquiry in the FBI's history. Its name stands for "Pentagon/Twin Towers Bombing Investigation". The investigation was launched on September 11, 2001, and involved 4,000 special agents and 3,000 professional employees.[2][3]