Mexican authorities have arrested a local police chief over suspected links to the killing of three women and six children last month, according to an official.
Suspected drug cartel hitmen shot three US mothers, twin babies and four other minors as they travelled along a dirt road in the northern Sonora state on November 4.
Seven other children survived by hiding by the roadside.
Several Mexican media outlets reported that Fidel Alejandro Villegas, police chief in the neighbouring state of Chihuahua, has been held on suspicion of involvement in the crime.
The reports said he is suspected of having ties to organised crime. Details of his alleged role in the killings were not clear.
A federal official confirmed the arrest of Villegas to the Reuters news agency. It follows the detention of other suspects earlier in the investigation.
Mexican officials believe the women and children were killed after becoming caught up in a dispute between local drug cartels battling for control of the area.
The group belonged to the LeBaron family - a breakaway Mormon community that settled in the hills and plains of northern Mexico decades ago.
Under pressure from the Trump administration, Mexican President Andres Manuel Lopez Obrador sought US cooperation in the case, inviting the FBI to help in the investigation.
Agencies contributed to this report
