Twenty-two people were arbitrarily executed by federal police on a ranch in Mexico last year, according to the country’s human rights commission.
The commission’s president Luis Raul Gonzalez Perez said the investigation revealed a range of human rights abuses on the part of government forces in the western state of Michoacan.
The government said the dead were drug cartel suspects hiding on a ranch in Tanhuato, near the border with Jalisco state, but refused to release the victims' post-mortem reports.
One police officer was killed in the confrontation on May 22 2015.
The government’s transparency watchdog earlier said there was no evidence to indicate human rights violations.
Police used a Black Hawk helicoper for the operation, firing a reported 4,000 rounds into the ranch.
The report found that police moved bodies and weapons to cover up arbitrary killings, while two bodies were burnt and two suspects were tortured while in custody.
Mr Gonzalez Perez said: "The investigation confirmed facts that show grave human rights violations attributable to public servants of the federal police."
However, Mexico's national security commissioner, Renato Sales, rejected the report's findings, stating the use of weapons was "necessary and proportional".
