Seventeen months after the disappearance of 43 Mexican students triggered a wave of protests that severely damaged President Enrique Peña Nieto's international image, the case continues to generate deep discomfort for the government.
On Tuesday, a group of international forensic experts rejected the government investigation's conclusion that the students were killed and incinerated in a garbage dump hours after they were attacked in the southern city of Iguala by municipal police, apparently in league with a local drug gang.
The Argentine Forensic Anthropology Team said their investigation of the dump, which is near the town of Cocula that is adjacent to Iguala, "does not support the hypothesis" that there was a fire of the size or duration that would have been required to incinerate the students. To reach this conclusion they carried out meticulous tests on a range of biological and non-biological matter at the site, including plants, insects, and debris.
The team also discounted that human remains of 19 people found in the area had anything to do with the disappearance of the students on September 26, 2014.
"There are no scientific elements to link the remains found in Cocula with the students from the Raúl Isidro Burgos Normal School in Ayotzinapa," Mercedes Doretti, who leads the team, told a packed press conference.
The anthropologists also stressed that there was no evidence to associate the dump with the bone fragment that has so far provided the only positive match yet with one of the missing students. The government said it found the fragment among the contents of several plastic bags discovered in a river that were filled primarily with ashes collected from the remains of the huge funeral pyre on which it claimed the students' bodies had been burned.
Tuesday's report is the second independent study to demolish the so-called "historical truth" of what happened to the students that was laid out by the then Attorney General Jesús Murillo Karam in November 2014. Murillo Karam claimed that several detained members of a local drug gang called Guerreros Unidos had confessed to burning the bodies and dumping the ashes.
Though the parents of the missing students never accepted this version of events, government officials dismissed their skepticism as the product of their pain.
This became much harder to do after a special group of international experts convened by the Inter-American Commission on Human Rights, known as the GIEI, released a 500-page report in September that described the garbage dump story as "scientifically impossible."
The government sought to downplay just how devastating this conclusion was for its investigation by promising to carry out a new new forensic analysis that it said would clear up any doubt. It reiterated its commitment to this new study in a statement released on Tuesday saying it would provide "a conclusive opinion" on the issue.
This appeared to squash the hopes of relatives of the missing students present at the Tuesday's press conference who said they wanted the government to dispense with the idea of an additional study.
"That would be another effort to distract attention," said Melitón Ortega, one of the parents. "The government has tried to dupe the parents from the start with its campaign of lies."
Vidulfo Rosales, the human rights lawyer who has represented the parents from the start, also said he hoped the government would leave behind the mass incineration version once and for all. "Going back to the Cocula dump is a waste of resources," he said.
For some observers, the renewed spotlight on the tip highlights the way the government has not been held accountable for an investigation that was obviously inadequate.
"What Mexico needs isn't just an investigation into the whereabouts of the disappeared students, but also an investigation of the authorities who produced the unsubstantiated official version of events," José Miguel Vivanco, America's Director of Human Rights Watch said in response to the forensic team's study.
"The so-called "historical truth" on the fate of the 43 disappeared students was no more than a fiction," he added. "The authorities involved must be made to answer for their role in perpetuating impunity."
Despite the continued controversy over the "historical truth", there are some signs that the government is now making more effort to find out what really happened to the students.
In November it relaunched its probe with a new investigative team within the attorney general's office. This was a direct result of the international pressure generated by the release of the Inter-American Commission GIEI's report two months before. As well as discounting the garbage dump theory, that report also dismissed the government's claim that the students were targeted because they were mistaken for a rival drug cartel.
"This is a fresh team with no preconceived ideas and that is good for doing things differently, in a more direct way, and for reaching agreements," Carlos Beristáin, who heads the GIEI, told VICE News.
Ángela Buitrago, another member of the five-member GIEI, said that the new investigation is finding more elements to support the so-called "fifth bus" hypothesis that was first posed in their September report.
This suggested the students — who had gone to Iguala to commandeer passengers buses to use in political protests — may have been attacked because they mistakenly took a vehicle that was packed with hidden opium paste.
Iguala is a key entry and exit point for the isolated mountainous region where large amounts of opium poppy is cultivated in a patchwork of territories controlled by various different criminal groups.
Buitrago also said that she was confident that the team is getting closer to finding out what happened to the students after they were abducted.
"We think we're pretty close to finding out where they were taken," Buitrago said, though she declined to give any details.
The experts are due to release their second and probably final report at the end of April.
Since they published their first damning revision of the government's investigation, the experts have complained that their personal reputations and work have been consistently pilloried by well-known columnists in the media who have a tendency to give voice to interest groups within the government.
"When a report cannot be questioned with data, like nobody has, what people do is question its legitimacy," Buitrago said. "But we believe they do this because we are closer to the truth."
Meanwhile, although the parents of the missing students do acknowledge that communication with the government investigators has improved, they continue to insist that the investigation will not be complete unless full access is given to the soldiers who were in Iguala during the attacks, and yet did not intervene to stop them. Such interviews have been repeatedly ruled out by the army top brass.
The parents also complain that not enough is being done to find the missing students.
"They are still saying nothing about our children," Joaquina García, one of the mothers, told VICE News, adding that the parents are still travelling the country in the hope of picking up some clue of where the students might be.
The attorney general's statement responding to this week's new forensic report did not mention the search for the disappeared students.
"The case is not closed," the statement said. "The investigation continues, and will continue, until the last person responsible has been presented before a judge."
Follow Melissa del Pozo on Twitter: @melissadps