MATAMOROS, Mexico — Authorities were investigating on Friday a possible police connection to the killing of three U.S. citizens visiting their father in Mexico who were found shot to death along with a Mexican friend more than two weeks after going missing.

Parents of the three siblings, whose bodies were identified Thursday, have said 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 wracked 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.

The father of the three Americans, Pedro Alvarado, identified his children from photographs of the bodies showing tattoos, Quintanilla told Radio Formula.

Clothing found with the bodies also matched that of Erica Alvarado Rivera, 26, and her brothers, Alex, 22, and Jose Angel, 21, who disappeared Oct. 13 along with Jose Guadalupe Castaneda Benitez, Erica Alvarado’s 32-year-old boyfriend.

Each was shot in the head and the bodies were burned.