A friend of mine is a sniper for the FBI, attached to DoD SpecOps teams. Before that, he was a BigLaw associate working on securities compliance matters.
People's careers take the darndest turns. As for how to get there, I haven't the foggiest idea - I just wanted to throw my oddball story out there. That said, my friend told me that his class at the academy was relatively lean on lawyers and accountants, but loaded with police officers. I suspect the FBI alters their criteria depending on current needs, and for the moment anti-terrorist efforts are key.