The National Human Rights Commission has opened an investigation into the killing of 42 people last week in a confrontation with the federal police. The authorities have described those killed as members of a powerful gang who fired repeatedly on officers during a three-hour battle in Michoacán State in southwestern Mexico. Security analysts and commentators have raised questions about the lopsided toll; there were only three arrests, no wounded survivors, and only one police officer killed. Last year, the killing of 22 people in a confrontation with the army led to charges against several soldiers when the commission determined that most victims had been summarily executed after an initial gun battle.