When U.S. Special Operations forces raided several houses in the Iraqi city of Ramadi in March 2006, two Army Rangers were killed when gunfire erupted on the ground floor of one home. A third member of the team was knocked unconscious and shredded by ball bearings when a teenage insurgent detonated a suicide vest. Who, the relative asked, was this man — the one represented by a blue dot and nearly killed by the suicide bomber? After some hesitation, the military briefers answered with three letters: FBI. The wounded agent in Iraq was Jay Tabb, a longtime member of the...