The Mexican government’s human rights body has accused police of killing 22 people in extrajudicial executions in a raid on a drug cartel last year.
One police officer and 42 suspects were killed in the raid on a ranch in Tanhuato, Michoacan state, on 15 May.
Officers said they had returned fire in self-defence but the high death toll aroused suspicions.
The National Human Rights Commission (CNDH) says police attempted a cover-up. Police dispute the findings.
Human rights groups in the country have long been demanding an improvement in policing standards and an end to arbitrary killings.
Previously, the government said there had been no human rights violations during the raid on the ranch.
They said a war was being fought between two local drug gangs and those killed were believed to be members of one of the cartels.