The first body belonging to a victim of the May 26 mass kidnapping at a Mexico City bar has been released to relatives, the Federal District Attorney's Office said.

Investigators found the remains of 13 people, all of them abducted from the Heaven bar in Mexico City's upscale Zona Rosa district, in a clandestine grave in the city of Tlalmanalco last month.

The other bodies are still at the coroner's office in Mexico City, the DA's office said.

Relatives of the victim whose body was released went to the office of the special prosecutor for kidnappings to be briefed on "the physical and scientific evidence used to identify their family member," the DA's office said.

A team of prosecutors and specialists "explained to them in a clear fashion the results of the medical, anthropological, criminological, dental, genetic and photographic techniques used, allowing them to know with certainty that it was their daughter," the DA's office said.

Relatives asked officials to complete "all the procedures so they could receive the body, which was released to the deceased's father" on Tuesday, the DA's office said.

The bodies were found on the La Mesa ranch in Tlalmanalco, a city in Mexico state.

Federal authorities found the mass grave - covered with cement, asbestos and lime - during an unrelated firearms investigation.

Seven people are in custody in connection with the mass abduction and murders, officials said.

The mass abduction and murder was carried out in reprisal for the May 24 killing of Horacio Vite Angel, a reputed member of the La Union de Insurgentes drug gang, by the rival La Union de Tepito group, officials said.

One of the people taken from the bar and killed, 16-year-old Jerzy Ortiz, was the son of Jorge "El Tanque" Ortiz Reyes, the jailed boss of La Union de Tepito, officials said. EFE