An official probing the massacre of 72 suspected illegal migrants in north Mexico went missing as media reported relatives of the Guatemalans murdered in the case have received ransom calls.

Roberto Suarez, who was one of the first to find the bodies in the northern state of Tamaulipas, was missing, but there was no evidence that he has been murdered, Mexico's President Felipe Calderon said on Friday.

Speaking at a press conference, Calderon said, "We are using all the resources available to investigate this case."

A total of 31 corpses had been identified till Friday, including 14 Hondurans, 12 Salvadorans, four Guatemalans and a Brazilian, Mexican broadcasters said, citing the Tamaulipas authorities.

Meanwhile, an official from the State Prosecutor's Office (PGE) told Xinhua that the disappearance of Suarez since Wednesday morning will not hold back the investigation.

Ruben Dario Rios, who is working for the PGE office in Tamaulipas, said by phone that the corpses have been sent to the eastern Mexican city of Reynosa and were at the disposal of the Attorney General's Office there.

In a related development, Daniel Boche, a relative of the Guatemalan victims, told media that he had received telephone calls demanding 2,000 U.S. dollars for getting back his son and two sons-in-law, according to news reaching here.

His 17-year-old son Gelder Lizardo Boche Cante and sons-in-law -- Gilmar Augusto Morales Castillo, 22, and Hermelindo Maquin Huertas, 24 -- were murdered on Tuesday night, he said.

Foreign Ministry Spokesman Andrea Furlan named a fourth victim as Agustin Hernandez Santos, adding that Guatemalan consuls in the northern Mexican cities of Tijuana and Monterrey were working with the authorities in Tamaulipas to identify the dead.

The 72 bodies were found on a ranch in Tamaulipas state bordering the southern U.S. state of Texas late Tuesday after a clash between Mexican marine troops and suspected drug-cartel gunmen that left one marine and three suspects dead.

It appeared to be the largest drug-cartel body dumping ground found in Mexico since an offensive was launched against drug trafficking in late 2006.