Mexican human rights investigators on Tuesday interviewed employees at an import car lot where the parents of three young Americans shot to death in Mexico say they found their vehicles.

Inside a tall iron gate, a team of five from the National Human Rights Commission was seen conducting interviews and taking photographs at the property in downtown Matamoros. The Alpha car import business is owned by Luis Alfredo Biasi, social welfare secretary for the city of Matamoros.

Before their bodies and that of a friend were found last Wednesday outside the city, Erica, Alex and Jose Angel Alvarado Rivera, siblings from Progreso, Texas, had not been seen since Oct. 13 when they were at a taco restaurant near the Los Indios international bridge on the Texas-Mexico border.

Their parents have said witnesses told them that armed men wearing uniforms belonging to a special Matamoros police unit called "Grupo Hercules" and possibly marines took the three young people away along with Erica's boyfriend, Jose Guadalupe Castaneda Benitez.

Pedro Alvarado, the siblings' father, said he found Erica's Jeep Cherokee and Alex's Tahoe at the Alpha lot following their disappearance.

After investigators departed the car lot Tuesday, the manager said he had given a statement to authorities and declined to say more. He insisted that his name not be used, saying he feared reprisals. The investigators also declined to comment on the case because it remained under investigation and asked that their names not be used for security reasons.

Later Tuesday, the National Human Rights Commission released a statement saying it had started investigating the siblings' disappearances in October and is now looking into their homicides as well.

The restaurant where they were last seen, La Curva Texas, is in a simple wood frame building and is known for its grilled meat tacos. It sits under an overpass in the median of the highway connecting Matamoros and Reynosa two miles from the U.S. border.

Raquel Alvarado, mother of the dead siblings, has quoted witnesses as giving this account of their disappearance: Erica Alvarado and Castaneda were eating at the restaurant around midday Oct. 13. Erica had asked her brothers to bring her Jeep there so all three could return together to Progreso, and when the brothers arrived they found armed men pushing and beating their sister and Castaneda. After the brothers intervened all four were taken away.

The mother said the sibling's father, her former husband, confronted someone at the car lot after finding both vehicles there. He was allowed to drive the Tahoe home and the Jeep was towed to his home in the nearby Mexican town of El Control. She said it was a member of Hercules who gave her ex-husband the key to the Jeep.

On Oct. 14, Pedro Alvarado went to the offices of the state human rights commission in Matamoros to report his children as missing, said Jose Javier Saldana Badillo, the state commission's regional delegate. On Oct. 15, Saldana passed Alvarado's complaint to the national commission because at that time Alvarado had said marines or soldiers or some combination of both had taken the four away.

"When he came to us he indicated that it had been members of the Mexican army or the marines, but he didn't say it was Grupo Hercules," Saldana said. He speculated that so soon after the disappearances, Alvarado hadn't found the witness who told him it was Hercules.

Saldana said the team from the National Human Rights Commission began interviewing people in the area on Monday.

Mayor Leticia Salazar and Biasi, the city official who owns the car lot, were in El Control for an event Oct. 13 shortly before the Alvarado siblings and Castaneda were taken from the restaurant. Neither has responded to requests from The Associated Press for an interview.