MEXICO CITY—In September 2014, 43 university students disappeared in Guerrero state in southern Mexico. The Mexican government has maintained that a drug cartel murdered the students and burned their bodies at a trash dump. But forensic investigators and human rights groups were doubtful, citing gaps in the evidence and a federal investigation that they contend fell short of international standards. Now, a renowned fire scientist says his latest experiments rule out the government’s explanation once and for all.
Using pig carcasses as a proxy for human bodies, José Torero, a fire scientist at the University of Queensland, St. Lucia, in Brisbane, Australia, incinerated up to four pigs at a time and determined that the inferno necessary to consume 43 bodies could not have occurred at the dump. “José knows what he’s doing,” says John Lentini, an independent fire investigator in Islamorada, Florida, who wasn’t involved in the research but has participated in other high-profile cases. “It doesn’t make any sense that you can make 43 people disappear like that.”
Torero’s experiments “are one more element that says the so-called ‘historical truth’”—how a former attorney general labeled the government’s theory of the crime—“is impossible,” says Francisco Cox Vidal, a lawyer and member of an expert group (known in Spanish as the GIEI) convened by the Inter-American Commission of Human Rights in Washington, D.C., to examine the disappearance and the official inquest. Deputy Attorney General for Human Rights Eber Betanzos Torres did not respond to requests for comment.
The missing students studied at the Ayotzinapa Normal School, a rural teacher’s college near Tixtla, Guerrero. On 26 September, a larger group of Ayotzinapa students hijacked commercial buses to travel to a protest in Mexico City, a common practice at the politically active school. According to confessions from members of the Guerreros Unidos cartel, the gang, abetted by local police, ambushed the students, possibly mistaking them for members of a rival cartel. Some students were killed by gunfire, some escaped, and 43 were kidnapped and allegedly executed. Cartel members said they incinerated the bodies in a municipal dump outside the town of Cocula. Six weeks later, federal investigators said they had found bags of human remains, burned to ash, in the dump and in a nearby river.
The ash was sent to a lab at the University of Innsbruck in Austria. Almost all the organic matter in the ash had burned away, but the lab eventually recovered DNA of two missing students in remains from the river. In April 2016, the lab announced that it had not been able to link any of the other 41 students to the remains, nor to hair and clothing samples recovered from the dump.
From the start, doubts plagued the Attorney General’s reconstruction of events. Basic facts, even the number of buses commandeered, differed in the official reports and the escaped students’ own accounts. (GIEI has theorized that unbeknownst to the students, one of the hijacked buses may have been used for drug running.) A group monitoring the investigation on behalf of the victims’ families has questioned the provenance and chain of custody of the ash bags. Human rights groups also suspect that the cartel members’ confessions were extracted under torture. Faced with international criticism, the Mexican government agreed to allow GIEI to investigate.
We should stop looking into the dump because that’s not what happened.
The expert group called on Torero. Born in Peru and trained at the University of California, Berkeley, Torero has investigated many high-profile fires, including those that brought down the Twin Towers. The cartel members had testified that they incinerated the bodies on a pyre of wood and tires in the open air. Torero’s calculations suggested that fully incinerating 43 bodies in the manner the cartel described would have required a staggering amount of wood: between 20,000 and 40,000 kilograms. He also doubted that it would be possible to nearly eliminate organic matter from the remains with an open-air fire, rather than with a furnace. And when he visited the Cocula dump in July 2015, he saw no evidence of a massive fire. He concluded that it was impossible the students had been burned there.
In an 8 June report, the Attorney General’s office called for experimental verification. Torero independently took up the challenge. He and a dozen students simulated the alleged pyres at Cocula in a field at his university’s Gatton campus, outside Brisbane. They used bone-dry wood, stacked precisely, and left out tires, which would have made the fire less efficient. The experimental set-up, Torero says, represented “the ideal scenario.”
His team systematically burned pig carcasses. Even when using 630 kg of wood for a single 70-kg pig, 10% of the pig’s flesh remained after the fire burned out, Torero told Science. Forty-three bodies of a similar weight, therefore, would have required over 27,000 kg of wood, and organic matter would have survived the fire. Even if the cartel had been able to find that much wood in Cocula, the intense bonfire would have scarred nearby tree trunks, Torero says. Visiting the dump 10 months after the disappearances, he saw no such scars.
Torero also burned up to four pig carcasses at once to explore whether body fat would fuel the fire and promote total incineration. Each added carcass weakened fire intensity, the team found. Burning 43 bodies together, therefore, would require much more wood than burning each separately. “Bodies are a large percent water,” says Lentini. “They’re not great fuel.”
Torero plans to submit his findings for peer review in the fall. In the meantime, he hopes his experiments will prod investigators in the grisly case to move beyond Cocula. “We should stop looking into the dump,” Torero says, “because that’s not what happened.”
*Update, 15 September, 10:21 a.m.: On 14 September, Tomás Zerón de Lucio resigned as the head of the Attorney General's criminal investigations division. He was a key figure in the government's investigation, and the parents of the Ayotzinapa victims had long called for his dismissal.
Hours later, President Enrique Peña Nieto appointed Zerón de Lucio as the technical secretary of the National Security Council.
*Update, 14 September, 1:37 p.m.: On 13 September, the Attorney General’s (AG's) office announced that it is expanding its investigation to examine the possible role of the state and federal police in the students’ disappearance. In a Spanish-language interview with Reuters, the new head of the investigation made no mention of the Cocula dump, nor of Torero’s experiments.
A spokesman for the AG contacted Science in the evening of 13 September. In a statement, he said a panel of fire experts convened by the AG’s office in the spring had reached “the majority conclusion” that “a controlled fire had occurred in the Cocula dump, in which it was possible that human bodies had been burned.” José Torero participated in that panel, along with five other fire experts. The panel was confidential and its methods were never publicly released. Torero declined to discuss it with Science because he had signed a confidentiality agreement.
When asked if the AG will continue to investigate the Cocula dump as a probable crime scene, the spokesperson said the AG’s office objectively evaluates all valid evidence and pursues all promising leads.
Listen to a podcast on this article with author, Lizzie Wade.
For more coverage on this topic, check out our Forensics feature package.
*Update, 22 May 2018, 10:10 a.m.: The results of José Torero's study have now been published in the Fire Safety Journal.