Mexico's national human rights commission has revealed new evidence pointing to the direct involvement of federal police agents in the disappearance of 43 student teachers from the southern city of Iguala in 2014.
The head of the commission's investigation into the students' case said it has a credible witness who saw two federal police agents watch municipal police abduct between 15 and 20 of the students from one of the buses that the group was traveling in.
José Larrieta Carrasco said the witness overheard one of the agents asking a municipal officer overseeing the operation what was going on. He was told that the students were being taken to another town where el patrón, meaning the boss, "will decide what to do with them." The witness heard the federal police officer respond "ah, okay," and then saw him and a partner stay to watch the students being loaded into municipal police trucks and driven off.
Larrieta said his team had corroborated and cross-checked other information in the witness's account, and come to the conclusion that it was probably "trustworthy and true."
The allegations of federal police involvement from the national rights ombudsman's office, that has been notably cautious in its pronouncements about the case up until now, represent a potentially major blow to the government's investigation into the horror that unfolded after dark on September 26, 2014.
The official probe has always put the blame on members of a local drug gang, called Guerreros Unidos, as well as corrupt municipal officials and police officers from Iguala and the neighboring town of Cocula.
Government officials have also repeatedly rejected allegations from national and international human rights activists that they are covering up possible federal involvement. Up until now, however, the evidence of federal collusion in the public domain has been largely limited to proof that federal authorities were closely monitoring the students' movements.
The students had gone to Iguala from their famously radical Ayotzinapa teacher training college, about two hours drive away, on a mission to commandeer passenger buses to use in a later protest. They were attacked as they sought to return to the college in those buses.
Their disappearance sparked a wave of outrage and protests around Mexico, and severely damaged President Enrique Peña Nieto's international image. Though the huge marches have since faded, the case remains a sore point for the government given the large number of human rights groups and institutions that have questioned the quality of the official investigation.
The tension around the case has risen sharply in recent weeks in the run up to the release at the end of the month of a much-anticipated report by a group of experts convened by the Inter-American Commission on Human Rights.
In a previous report last September, the experts discounted the official investigation's conclusion that the students were incinerated a few hours after they were abducted in a garbage dump outside of Cocula. The government recovered bags full of ashes and a few bone fragments from a river in the area. It said the remains had been taken there from the tip.
The government responded to the September report from the Inter-American Commission's experts with promises to revise the garbage fire hypothesis. With the new report weeks away some officials and government-friendly media have begun pushing the old story once again.
The new evidence presented by the normally-cautious national human rights commission on Thursday, meanwhile, also indirectly questions the garbage pyre version.
This is because the eyewitness both heard the police saying the students were being taken to a town called Huitzuco, and watched them being driven off in that direction. Huitzuco and Cocula are located on opposite sides of Iguala.
It also appears that the group of students on the particular bus in question included Alexander Mora. A bone fragment later identified as belonging to Mora was found in the bag of human remains recovered from the river that the government said came from the garbage tip. No other student has been positively identified apart from him.
Follow Jo Tuckman on Twitter: @jotuckman