Mexican police arbitrarily executed nearly two dozen suspected gang members on a ranch last year, the government's National Human Rights Commission said.
It has been described as one of the worst abuses by security forces in a decade of drug violence.
In May of last year, federal police ambushed suspected members of the Jalisco New Generation Cartel (JNG) holed up at Rancho El Sol near the small town of Tanhuato in the violent western state of Michoacan and killed 42 men.
According to the president of Mexico's National Human Rights Commission Raul Gonzalez, police lied about their role during the incident, moved seven bodies and shifted weapons to manipulate the scene.
Investigators were also unable to clarify how another 15 of the victims were killed.
Only one policeman died in the fight, in which police backed by a Black Hawk helicopter attacked the cartel, a kill rate way higher than international norms, but not uncommon in Mexico's drug war.
Only one injury was reported.
The one-sided toll was one of the highest since President Enrique Pena Nieto took office in 2012, pledging to end years of violence.
It is a fresh blow to his government with the president's approval rating at an all-time low over perceptions he has not tackled rampant crime and corruption.
Speaking at a news conference in Mexico City, National Security Commissioner Renato Sales defended the officers.
"In our opinion the use of arms was necessary and proportionate to the real, imminent aggression and it had no rights basis. That is to say that in our consideration they acted legitimately in self-defence given that the aggressors had weapons that are exclusively reserved for the armed forces, such as a Barrett and some RPG launchers", he said.
Earlier this year, the Open Society Justice Initiative, a private human rights body, said incidents including Tanhuato constituted crimes against humanity.
It said the International Criminal Court should step in if Mexico fails to resolve such cases.
Mexico's federal police, the army and the navy have long been implicated in abuses since a drug war that has claimed more than 100,000 lives began in 2006.
Police killed 17 people for every officer lost in gunbattles in 2014, according to a study by Mexico's National Autonomous University, a number the study said was consistent with excessive force.
And in shootouts involving Mexican police between 2007 and 2013, the number of people killed for each person injured rocketed from 1.6 to more than 20, the study said.
