A mob had gathered by the time the F.B.I. agents arrived at the house where an assassin's bomb killed nine people last year, narrowly missing a deputy prime minister. Fearing their own lives might be at risk, the agents gave themselves no more than 30 minutes to collect evidence.
As agents worked inside the house, an Iraqi police commander outside ordered the arrest of a man on the fringe of the crowd, according to American agents who were at the scene. The man later confessed to complicity in the attack. The case, if it could be called that, was quickly closed.
But it was never really clear to American investigators whether the man was actually guilty, or whether the Iraqi police coerced his confession. As an attempt at Iraqi-American cooperation in law enforcement, the investigators said, the episode was clearly disappointing.
The attempted assassination of the deputy prime minister, Salam al-Zubaie, in March, is just one of many episodes American law enforcement agents recounted as they described their often frustrating efforts to bring the rule of law to Iraq. In other examples, a suspect arrested after a brazen, deadly armed robbery managed to escape, and officials implicated in the abuse of prisoners have gone unpunished. Investigators cannot easily visit crime scenes, and judges live in fear and hiding.