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An arrest has been made in the murder of nine American Mormons in Mexico.

On Monday, three American women and six of their children were murdered in a highway ambush believed to be the work of local drug cartels. The women and children who were murdered–including a set of eight-month-old twins–belonged to a Mormon community living in the Sonora region of Mexico, about 70 miles south of the Arizona border. The victims’ bodies were found in burned-out SUVs, having been shot to death.

“The state prosecutor’s office in Sonora state, where the killings occurred, said in a statement posted to social media that agents detained a heavily-armed man close to where the attacks took place, and that ‘the possibility’ that he might have participated in the massacre was ‘being analyzed,'” CBS News reported.

The FBI is investigating, and American and Mexican authorities aren’t sure whether the families were targeted by the cartels, or if the killings were random. Mexican officials said that local cartel members might have mistaken the families’ SUVs for those of rival gang members.

Some of the three women’s children survived the attack. Devin Langford, a 13-year-old whose mother was killed in the ambush, hid six of his siblings in the bushes before walking twelve miles for help. Christina Langford Johnson, one of the murdered women, hid her 7-month-old daughter in the floorboards of her SUV before being shot. The baby was found the next day, unharmed.