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 country is already convulsed by the case of 43 students from a teachers college in the southern state of Guerrero, their disappearance blamed on a mayor and police working with a drug cartel. Fifty-six people are under arrest, including dozens of police officers.

In a separate case in June, soldiers killed 22 suspected gang members in Mexico state, then altered the scene and intimidated witnesses to hide the fact that most of the dead were executed after they surrendered, a National Commission on Human Rights report said last week. Three soldiers face murder charges.

“We will apply the full force of the law and zero tolerance,” Tamaulipas Gov. Egidio Torre Cantu said of the latest case, lamenting the death of the three Americans and a Mexican citizen, even though their identities had yet to be confirmed by DNA.

Presidential spokesman Eduardo Sanchez declined to comment when asked about the newest case.

“We have been in touch with Mexican officials … and will take appropriate action once more is known on the circumstances,” the U.S. Embassy said Friday in a statement.

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, most likely from lying in the hot sun for so long, Quintanilla said.

Tamaulipas authorities said it could take 24 to 48 hours for DNA tests to further confirm that the bodies were those of the Alvarado siblings, who were last seen in El Control, a small town near the Texas border west of Matamoros, about to return home to Progreso, Texas.

“They were good kids,” said an aunt, Nohemi Gonzalez. “I don’t know why they did that to them.”

The three siblings shared their mother’s modest brick home on a quiet street in Progreso less than three miles from the border. Erica, who has four children between the ages of 3 and 9, had been scheduled to begin studying next month to become a nursing assistant.

Brothers Jose Angel and Alex had been set to make their annual pilgrimage to Missouri as migrant farm workers more than a week ago, Gonzalez said. When they weren’t on the road, they divided their time between their mother’s house in Texas and their father’s in Mexico.

On Sunday, Oct. 12, Erica drove her black Jeep Cherokee across the border to El Control. She dropped it at her father’s house and went to visit with her boyfriend.

Her mother, Raquel Alvarado, had told her to be back in Progreso by early Monday morning, because Raquel had to work and Erica’s kids had to get to school. Raquel put the kids to bed Sunday night and awoke at 4 a.m. to see Erica was not home. She began calling her daughter’s cellphone and continued through the morning. “I’m always worried about her when she goes over there,” the mother said.

Around 1 p.m., she reached her former husband. He told her Erica had called her brothers and asked them to bring her Jeep to a roadside restaurant under a bridge near El Control where she was eating with her boyfriend. One brother drove her Jeep and the other drove his Chevrolet Tahoe because they all planned to return to Progreso from there.

According to Raquel Alvarado, witnesses told family members that the brothers arrived around 12:30 p.m. and saw members of the police unit called Hercules pushing their sister and Castaneda and hitting Erica. When the brothers intervened, the police took all four of them, along with their vehicles. The witnesses said the armed men identified themselves as members of the Hercules unit and warned against intervening.

The Alvarados say they later found their children’s cars at an import car lot belonging to Luis Alfredo Biasi, Matamoros‘ director of social services. Quintanilla could not confirm that. Biasi did not immediately respond to a request for comment from The Associated Press.

Mayor Leticia Salazar officially introduced Hercules in September as a group with particular skills to confront crime in high-risk operations, according to a press release.

City Clerk Joe Mariano Vega, who was identified in the release as the group’s commander, said in an interview earlier this year that Hercules was comprised of former marines and soldiers who policed hot zones for crime in the city’s neighborhoods.

Neither Salazar nor the city’s spokeswoman returned messages seeking comment.

Quintanilla said he saw no reason so far to interview Salazar or Biasi in the Alvarado case.

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