Tamaulipas state investigator Raul Galindo Vira would only confirm that four bodies had been recovered yesterday and declined to discuss who they might be.
A second state official said investigators were trying to determine if the dead include three siblings from Progreso, Texas, who disappeared with a fourth person Oct 13.
The official, who said the bodies were badly decomposed, insisted on speaking anonymously because he was not authorized to speak to the press.
Alvarado said witnesses saw armed men take her daughter, Erica Alvarado Rivera, 26, and her sons, Alex, 22, and Jose Angel, 21, in El Control, a small town near the Texas border west of Matamoros. The three were visiting their father in Mexico.
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According to Alvarado, her daughter, Erica, mother of four children aged 3-9, drove her black Jeep Cherokee across the border Oct 12 and dropped it at her father's house in El Control.
She visited her boyfriend there and the next morning called her brothers to ask them to bring the Cherokee to a roadside restaurant where the couple was eating.
When Alex and Jose Angel Alvarado arrived to pick up their sister, they saw men "pushing their sister and her boyfriend and hitting her," Raquel Alvarado said.
The brothers tried to intervene, witnesses said, but were taken away with their sister and her boyfriend. Witnesses said the armed men identified themselves as Grupo Hercules, a police security unit for Matamoros city officials, and were traveling in military style trucks.
She said witnesses also saw federal highway police, "but no one did anything."
The Matamoros mayor's office and a spokeswoman for the city did not respond to requests for comment.
She said no one had told her until she arrived that four bodies had been found.