Mexican authorities are questioning nine police officers from the elite Hurcules cartel-busting unit in connection with the killing of three U.S. citizens on vacation in the country

Erica Alvarado Rivera, 26, and brothers, Alex, 22, and Jose Angel, 21, disappeared on October 13, 2014 while visiting their father in the border city of Matamoros with Erica Alvarado's 32-year-old Mexican boyfriend, Jose Guadalupe Castaneda Benitez. Their bodies were found burned, most likely from lying in the hot sun for so long, police said. Each had been shot in the head. Parents of the three siblings have said that witnesses reported they were seized by men dressed in police gear calling themselves Hercules, a tactical security unit in the violent border city of Matamoros heavily racked by cartel infighting. Nine of the unit's 40 officers are being questioned, Tamaulipas state Attorney General Ismael Quintanilla Acosta said. It would be the third recent case of alleged abuse and killings by Mexican security forces.