BENGHAZI, Libya — A month after the killing of the US ambassador ignited a public outcry for civilian control of Libya’s fractious militias, that hope has been all but lost in a tangle of grudges and egos.

Scores of disparate militias remain Libya’s only effective police force but have resisted government control, a dynamic that is making it difficult for either the Libyan authorities or the United States to catch the attackers who killed Ambassador J. Christopher Stevens.