Mexico's transparency watchdog agency has taken the government's side in denying an appeal to release autopsy reports on 42 suspects killed by federal police in 2015.

One police officer died in what authorities described as a gunbattle with drug cartel suspects in the western state of Michoacan in May, but the lopsided 42-1 death toll drew suspicion.

The National Institute for Information Access recently ruled against a freedom-of-information request filed by The Associated Press in October. The quasi-independent agency ruled the information should be kept as a state secret for five years.

The decision drew criticism from human rights groups.

In a report Wednesday, the Inter-American Human Rights Commission criticized "a gradual regression in terms of active transparency policies and public information regarding the deaths of civilians."