Mexican security officials are being questioned about their involvement in the deaths of three U.S. citizens who went missing during their visit to Mexico last month.

Officials in Mexico say that the three Americans, who lived in Texas, were fatally shot near the border of the city of Matamoros. Now, authorities are questioning a local police unit that may have been involved, the attorney general said, according to the New York Daily News.

Pedro Alvarado identified the victims as his three children, Erica Alvarado Rivera, 26, Alex, 22, and Jose Angel, 21. The siblings, along with Erica's boyfriend, Jose Guadalupe Castaneda Benitez, went missing on Oct. 13, while they were visiting their father in Mexico.

"They were coming back, stopped to eat at a place called Roasted Meat Curve, and from there they got taken away," Alvarado said, reports the Los Angeles Times. "I went looking for them in Matamoros, and they didn't turn up, until just yesterday. I identified the bodies."

Each was found with a gunshot in the head and their bodies were burned, most likely since they were placed in the sun, Attorney General Ismael Quintanilla Acosta said.

"The bodies are not recognizable, you can't see anything, not the face, just the clothes," Alvarado said, adding that he used pictures of their tattoos and their clothing to give a preliminary identification.

According to the parents of the siblings, witnesses reported that the victims were kidnapped by men dressed in police uniforms who identified themselves as "Hercules," an ad hoc police squad formed by the mayor of Matamoros. Witnesses also said that the three men and woman were taken into custody by the Hercules squad.

"We will apply the full force of the law and zero tolerance," Tamaulipas Gov. Egidio Torre Cantu said.

Nine members of the Hercules police force are being investigated in connection with the deaths.