Inquiries into the deaths of three American siblings in a northern Mexico border city continue to focus on potential government ties.

Mexico's National Human Rights Commission, an independent body that investigates alleged cases of abuse including by government entities, sent investigators to Matamoros this week in pursuit of answers in the case of the three Alvarado siblings whose bodies were found with gunshot wounds last week.

The border state of Tamaulipas has been convulsed by cartel-related violence for more than four years. Soldiers, marines, as well as federal and state police, long ago replaced local police forces rife with corruption.

But with the changes have come more violent confrontations. Last month, Mexican authorities reported killing 19 alleged gunmen in three separate clashes within a half hour between Matamoros and Reynosa.

"Regrettably, this wave that is growing in our country has led to marines or soldiers being enabled like municipal police," said Jose Javier Saldana Badillo, regional delegate for the Tamaulipas State Human Rights Commission in Matamoros, who took the initial complaint from the Americans' father before referring it to the national commission.

The parents of Erica, Alex and Jose Angel Alvarado Rivera, siblings from Progreso, Texas, said armed men dressed in uniforms identifying them as "Grupo Hercules," a specialized police unit in Matamoros took them and Erica's boyfriend from a taco restaurant near the border town of El Control, Mexico on Oct. 13. Their bodies were found on Oct. 29, in a rural area outside Matamoros.

City officials have said Grupo Hercules is made up of former soldiers and marines who have been vetted by state police to provide security for city officials and target crime in high-risk areas.

On Tuesday, the human rights investigators were seen conducting interviews and taking photographs at the Alpha import car lot in downtown Matamoros. The company, which has several lots around the city, is owned by Matamoros' social welfare secretary, Luis Alfonso Biasi. He was not present and it was not known whether he had been interviewed. He has not responded to requests for an interview.

The parents of Erica, Alex and Jose Angel Alvarado Rivera say that Erica's Jeep Cherokee and Alex's Chevrolet Tahoe were found locked inside the gates of the Alpha lot following their disappearance.

The manager of the lot declined to speak on the issue Tuesday after being interviewed by the human rights investigators. Sitting with an open Bible on his desk, he said he feared reprisals and asked that his name not be printed. He locked the door after a reporter exited.

In addition to the human rights commission, Tamaulipas state investigators have already interviewed nine members of Grupo Hercules, as well as the unit's operations director Matamoros city clerk Joe Mariano Vega and the city's public security director Juan Sanchez Alvarado.

On Monday, a spokesman for the Tamaulipas state attorney's office handling the case said if investigators find a reason to take statements from Mayor Lety Salazar or Biasi they will, but so far there was not.

The National Human Rights Commission said in a statement Tuesday afternoon that the Alvarados were "allegedly detained near the Los Indios international bridge, without being taken before a governmental authority or knowing who detained them."

The case follows two others recently in which government entities have been implicated. On Tuesday, federal authorities announced the arrest of the mayor and his wife from Iguala, Guerrero where 43 students from a teachers' college disappeared in September. Authorities have said the mayor ordered police to intercept the students so they would not interrupt his wife's speech. The students were then handed over to a drug cartel and not heard from again.

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 many of the dead were executed after they surrendered, a National Commission on Human Rights report said last month. Three soldiers face murder charges.