They say the 58 men and 14 women had come from South and Central America.

Their bodies were found on August 24, after a shoot-out between suspected drug traffickers and security forces.

One of the surviving migrants told police the group had been kidnapped and killed by an armed gang.

The unnamed witness told Mexican officials that he had come from Ecuador, while others in the group were from countries including El Salvador, Honduras and Brazil.

He said the migrants were shot after refusing to work for the gang. He had been injured but managed to escape and alerted marines at a nearby checkpoint.

The military sent troops to investigate. When they arrived, the gang reportedly opened fire.

The victims' identities and nationalities have not been confirmed, but at a news conference officials said they had been in contact with various embassies.