The women and children, dual citizens of Mexico and the United States, were traveling in sports utility vehicles on a remote country road when they were attacked by gunmen believed to belong to an organized crime group. Federal officials have said that they are investigating the possibility that the victims were mistaken for members of a rival group.

The F.B.I. has worked with the Mexican authorities in the investigation.

Julián LeBarón, a cousin of the victims, said that the arrest of the Janos police chief confirms that Mexican law enforcement often collaborates with organized crime. “It’s common knowledge down here that the police work with the criminals,” he said by telephone from Chihuahua.

“They have a monopoly on security and they get paid a wage for protection, and later we find out that they participate in the murder of women and children,” he said. “These people take resources to protect us and they are murderers themselves.”

Mexico is about to close its most violent year on record, with 32,600 murders reported through November.

The violence is proving to be the biggest challenge to President Andrés Manuel López Obrador, who has centered his crime-fighting strategy around a new law enforcement agency, the National Guard, and promised that an emphasis on social policies would address the root causes of crime.