PENTTBOM
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.[1][2]
https://en.m.wikipedia.org/wiki/PENTTBOM