MEXICO CITY (AP) — The bodies of three men suspected of participating in the massacre of 72 migrants last month were found by the side of a road in northern Mexico after an anonymous caller told authorities where to find the cadavers, federal officials said Monday.

Prosecutors’ spokesman Ricardo Najera said authorities have no information on who made the call, but in the past suspects in especially brutal killings that draw too much attention to Mexico’s drug gangs have been “handed over” to authorities.

Federal security spokesman Alejandro Poire said the bodies of the three men — along with two women not identified as culprits — were found by marines last week after an anonymous caller on Aug. 30 tipped authorities off to the presence of the bodies in Tamaulipas state. It was unclear why authorities waited almost a week to announce the discovery.

The caller said the dead people participated in the Aug. 23 massacre. Poire said a Honduran man who survived the slaughter and is currently under police protection in Mexico had identified the three men as having been among the killers.

A total of seven men have now been identified as suspects in the massacre. The only one still alive was caught in a raid on Aug. 24 at the site of the killings, and identified by a survivor. Three other suspects died in a shootout with authorities at the scene of the killings.

Najera said “the evidence and testimony suggest very strongly that the Zetas were involved,” referring to a violent drug gang.

Of the witnesses, only the Ecuadorean and Honduran men are confirmed to have survived.

While El Salvador’s president said there was a third survivor — a Salvadoran man now in the United States — Najera said “there is no evidence at all at this point that he was a witness to these events.”

And the Navy announced Monday it had found a clandestine grave with two bodies in Tamaulipas, not far from the massacre site. It was unclear if the grave was related to the massacre. Marines arrested four suspects at the scene on Sept. 3.

It would not be the first time in Mexico that suspects have apparently been gift-wrapped for authorities. In 2008, drug traffickers in Michoacan lobbed hand grenades into a crowd celebrating Mexico’s Independence Day, killing eight.

Within 10 days of the attack, federal police say an anonymous phone tip then led them to a house where they found three suspects tied up, blindfolded and whimpering. The men later claimed they were forced into confessing to the crime.

Najera defended authorities handling of the migrants bodies — some critics say they were insufficiently refrigerated — noting, “the transfer of the bodies was very difficult” from the remote ranch were they were found.