Federal police in Mexico murdered 22 suspected cartel members during a raid last year and manipulated the crime scene to hide the extrajudicial killings, according to authorities there.

Mexico's Human Rights Commission said Thursday that of 42 suspects killed in a May 2015 raid, near the small town of Tanhuato, more than half were arbitrarily killed by police, with many shot in the back, NPR's Carrie Kahn tells our Newscast unit.

The commission also accuses police of rearranging bodies and weapons to hide the manner of the deaths. Police are said to have burned two bodies and tortured two suspects. The commission reports that it couldn't reach any definitive conclusions about how 15 others were killed.

"The investigation confirmed facts that show grave human rights violations attributable to public servants of the federal police," Commission President Luis Raul Gonzalez Perez said at a news conference.

The raid on the Rancho del Sol, roughly 210 miles northwest of Mexico City, was considered one of the deadliest in recent memory. Reports at the time put the battle between police — backed by a Black Hawk helicopter — and the Jalisco New Generation cartel. One officer was killed during the incident.

The Associated Press summarizes the commission's understanding of events:

"Federal police had said they encountered a truck and took fire from its passengers before being led to the ranch."The commission's report said the government did not produce evidence supporting that account and it said ... officers started their assault at least an hour earlier than they maintained in reporting on the incident. ..."According to the agency's report, after the federal police officer was shot, police called for backup. Fifty-four more federal police officers arrived along with a helicopter."The helicopter fired some 4,000 rounds at the ranch house and a nearby warehouse, which caught fire. The helicopter was also hit by gunfire, the report said. One victim died of burns that the commission believes came after he was shot but still alive."In total, five people were killed by the helicopter, the commission found. One victim was hit by a bullet that entered around his left pectoral muscle and exited his groin, but there were no bloodstains on the jeans he was found wearing, the commission said."Thirteen of the 22 people the commission said were executed had been shot in the back, it said."

Security officials immediately disputed the commission's findings.

"The use of weapons was necessary and proportional against the real and imminent and unlawful aggression," said Mexican Security Commissioner Renato Sales as quoted by Reuters. "That is to say, in our minds they acted in legitimate defense."

The news service goes on to report that the commission's accusation is the latest in a series of suspected excesses by security forces in Mexico's drug war:

"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."

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