A police officer has been arrested in connection with the May 26 kidnapping of 13 people from a Mexico City bar and their subsequent murder, the district attorney said.

The suspect, Edgar Ernesto Gutierrez Vega, joined the municipal police force six years ago, Rodolfo Rios told a press conference, adding that the officer appears to have been tipping off drug dealers about planned busts.

Gutierrez's standard of living "did not correspond" to his salary as a police officer, the district attorney said.

The officer was not on duty at the time of the mass kidnapping, Rios said.

Six people are in custody and facing charges in the case, while seven others remain free, but subject to court supervision.

Judges are still deciding the status of Gutierrez and another suspect who has a record of arrests for drug trafficking, Rios said.

The bodies of the people snatched in May from the Heaven bar in Mexico City's upscale Zona Rosa district were discovered last month by federal authorities pursuing leads in a firearms case unrelated to the Heaven kidnappings.

Investigators found the mass grave - covered with cement, asbestos and lime - on the La Mesa ranch in Tlalmanalco, Mexico state.

Autopsy results and other forensic evidence indicates the victims were killed within hours of their abduction, Rios told reporters.

Authorities say 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.

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