One of the world’s leading experts in fire forensics says the Mexican government’s account of the fate of 43 students that went missing in September 2014 is “impossible”, adding to the suspicion that a state cover-up may be afoot. According to the Mexican attorney general, the students were murdered by members of a drug cartel before being incinerated at a trash dump, but Jose Torero from the University of Queensland says the evidence suggests otherwise.

The students from the Ayotzinapa Rural Teachers’ College had hijacked five buses in order to travel to a protest in Mexico City commemorating the massacre of up to 300 students in 1968. However, members of the Guerreros Unidos cartel later claimed to have ambushed the students in the town of Iguala with the help of local police, who were supposedly acting under orders from the local mayor.

The cartel members say they abducted and executed 43 individuals, later burning their bodies on a pyre made of wood and tires at a dump near the town of Cocula. Shortly afterwards, investigators uncovered bags of ashes in a nearby river.

The contents of these bags were completely charred, with virtually all of the organic matter having been burned away. As a result, scientists were unable to extract much DNA from the remains, and could only identify two of the missing 43 students from this grisly discovery. It is this detail that aroused the suspicion of Torero.

In an interview with Science, he explained that it is virtually impossible to eliminate all the organic material from a human body when burning it on an open fire. This is because bodies contain a large water content, and not enough fat to provide the fuel for such complete incineration.

To confirm this, he ran a series of experiments using pig carcasses, which he burned on a pyre made of tires and dry wood. His findings revealed that it takes around 27,000 kilograms (59,500 pounds) of wood to burn 43 bodies, and that even this still leaves around 10 percent of the organic material intact. As such, he began to suspect that the Ayotzinapa students must have been disposed of at a crematorium.

He then decided to visit the site of the alleged fire in order to look for signs that a major blaze had occurred. Given the scale of the inferno that would have been needed to reduce the bodies to ashes, many of the surrounding trees should have been permanently scarred, yet Torero found no such markings on the nearby vegetation.

As a result, he states that “the hypothesis that 43 bodies were burned in that dump is impossible,” placing yet more pressure on the government to uncover the truth.