PENTTBOM 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
September 11 attacks
September 11 attacks
PENTTBOM