MEXICO CITY — The police chief of a small town near Mexico’s border with the United States has been arrested on suspicion that he was involved in the massacre of nine women and children of a Mormon family last month, the Mexican authorities said Friday.
The federal authorities arrested Fidel Alejandro Villegas, the police chief in the town of Janos in the state of Chihuahua, as part of their investigation into the Nov. 4 attack in a remote region in the neighboring state of Sonora.
Mr. Villegas was arrested this week on suspicion of protecting organized crime in the region and of colluding in the killings, a spokesman for Mexico’s Public Security Ministry said.
He is the fourth person to be arrested on suspicion of participating in the murder of the three woman and six children, members of a binational Mormon community that farms land in northern Mexico. The case has reverberated across the border, with President Trump once threatening to designate Mexican organized crime groups as foreign terrorist organizations.
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.
But the National Guard proved to be of little help on the day the members of the extended LeBarón and Langford families were killed. The authorities did not arrive at the scene until 10 hours later, Mr. LeBarón said.
“I’ve been marching in Mexico and speaking out against the violence,’’ he said. “And I have never seen a case where a victim has had justice.” Mr. LeBarón’s brother was killed 10 years ago when he stood up to local gangs and nobody has been tried for that crime, he said.
“The justice system has collapsed,” he said. “The impunity rate is almost 100 percent.”