A report released by Mexico's national human rights commission has concluded that federal police "arbitrarily executed" 22 people on a ranch in the western state of Michoacán last year, and then tried to cover it up.
The report, released on Thursday, adds up to a particularly damning indictment of Mexico's near-decade long offensive against the country's drug cartels that has been accompanied by numerous allegations of human rights abuse.
The events took place May 22, 2015, during an operation to take control of a ranch near the town of Tanhuato that had been occupied by gunmen belonging to the Jalisco New Generation Cartel.
The operation left 42 alleged gunmen dead. One police officer also lost his life.
The lopsided death toll immediately raised suspicions of extrajudicial executions. Government officials repeatedly dismissed these by stressing the superior equipment and training of the police, who were also supported by helicopters.
Now the commission says it found clear evidence that the authorities deliberately killed 22 during the operation by using obviously excessive force, which included 4,000 shots fired from the helicopters. The investigation also details the way police violated the crime scene by putting guns beside the dead who were apparently unarmed when they were killed.
"As a result of the investigation done by this organization, based on technical and scientific tests ... we established facts that imply grave human rights violations attributable to public servants of the federal police," Raul González, the commission's president, told a news conference.
Mexico's national security commissioner, Renato Sales, held his own press conference that began before the commission had finished presenting its conclusions, in which he insisted the police had acted with reasonable force.
"In our minds they acted in legitimate defense," Sales said. "It was an unfortunate event in which, as well as a federal police officer, 42 presumed members of organized crime, who had refused to put down their weapons, also died."
Thursday's report represents an important blow to President Enrique Peña Nieto, who has struggled to convince the public and the international community that his government is willing to take human rights abuses seriously.
The shootout at Tanhuato came nearly a year after another incident in the town of Tlatlaya, in neighboring Mexico State. A human rights commission report into that incident found that at least 12 people were executed during an operation in which a military patrol stormed a warehouse filled with alleged gunmen, and several women.
Though activists insisted that the investigation should include the chain of command and evidence of a shoot-to-kill orders sent to the head of the patrol, international pressure did help ensure that eight soldiers were arrested in connection to the alleged massacre. The cases against all of them have since been dropped for lack of evidence.
Follow Jo Tuckman on Twitter: @jotuckman