After the Miami Shootout in 1986, the FBI invented modern ammunition testing and led the law-enforcement movement to .40 caliber. The FBI is very conservative, and has one of the best ballistics labs in the world.
With the amazing 21st-century advances in ammunition technology, many people have advocated a move back to 9mm. Recently, the FBI announced they were buying a new gun in 9mm. When the FBI makes a move like that, they have thought it through very thoroughly.
For the rest of the story: http://loadoutroom.com/12077/fbi-going-9mm-comes-science