Four family members, including three Americans, were shot in an attack in Durango, Mexico, that left three of the four relatives dead, officials said.

The three adults were all killed: two brothers who are U.S. citizens, Vicente Peña Rodríguez and Antonio Fernández Rodríguez, and their relative, who is a local resident, Jorge Eduardo Vargas Aguirre, the Durango Attorney General’s Office said.

Vicente Peña Rodríguez’s son, a minor, survived, and is in critical condition, the attorney general’s office said.

The attack unfolded on Friday night as the family was traveling in a GMC Yukon with an Illinois license plate, officials said. The SUV was found on the side of the Francisco Zarco highway.

Authorities are investigating whether robbery was a motive, according to the attorney general’s office.

Vicente Peña Rodríguez lived in Chicago and was in Mexico for a vacation with his teenage son, according to Chicago ABC station WLS.