Relatives of the 43 missing students hold signs that say “Where are they?” after listening to an expert group’s report presentation in Mexico City on April 24. (Marco Ugarte/AP)

A group of outside experts who have spent more than a year investigating the disappearance of 43 students discovered that authorities, including federal police and soldiers, were tracking in real time the movement of the students on the night of their abduction.

They identified a possible motive for the crime: The buses may have been used to traffic heroin out of the western town of Iguala, and the students may have inadvertently commandeered one of them, prompting a furious search for them. The experts documented torture of suspects, and they debunked the government’s central conclusion, denying that the students were burned to death in a remote trash dump in the mountains far from any homes.

But as the Interdisciplinary Group of Independent Experts — which includes prominent lawyers and prosecutors from across Latin America convened by the Inter-American Commission on Human Rights — wrapped up its work with the release of its second and final report on Sunday, it is for members a frustrating ending, both for the unsolved mystery that remains and the belief that the Mexican government impeded their investigation.

The crime against the 43 students at the teachers’ college in Ayotzinapa nearly two years ago has been the defining human rights case of President Enrique Peña Nieto’s administration. The circumstances confirmed so far have highlighted how government officials and police have been bought off by drug cartels and serve on their behalf. It has laid bare the incompetence of investigators, who have failed to protect crime scenes and properly handle evidence, and raised warnings of abuse with repeated allegations that authorities tortured their suspects.

Above all, the crime has called into question the will of federal authorities to uncover the truth about what happened to the students or whether they chose instead to invent a false narrative to put the case to rest.

“The government wasn’t cooperating, and that’s what should be under investigation,” said Joy Olson, executive director of the Washington Office on Latin America, who attended the report’s presentation in Mexico City. “What they’ve shown is that the official story about what happened is not sustained by the facts.”

She defended the quality of the group’s investigation and said whatever questions remain have more to do with the lack of assistance from the government.

“And that’s what makes it so damn sad today,”she added.

The Mexican government did not have an immediate response to the latest report, but Peña Nieto tweeted that he thanked the experts for their work and that the attorney general’s office would “analyze the complete report to enrich their investigation.”

Eber Omar Betanzos Torres, the deputy attorney general responsible for human rights, spoke in defense of the government on Sunday evening. He told reporters that the government gave the experts “full access” to information for “the development of their work.” Of the 941 requests for information, he said, about 85 percent have been fulfilled.

The experts said that months of government silence were followed by a dump of documents just days before their deadline, giving them too little time to analyze them.

The case has prompted widespread outrage and months of protests by hundreds of thousands of people. On Sunday, hundreds gathered in a university courtyard in Mexico City to hear the group’s conclusions. The crowd chanted from one to 43 before the event started.

Although the group’s presentation was largely a technical discussion of facts, the ultimate message was one of frustration about the lack of help from the Mexican government. Despite being invited by the government to investigate the case, the members were denied almost all access to the Mexican military, despite soldiers’ documented presence at several locations where the students were seen. Requests for documents languished for months, and an apparent smear campaign began against the group in parts of the Mexican news media. Such delay tactics and obfuscation have reminded many observers of the classic moves of the Institutional Revolutionary Party, or PRI, ­sometimes called the “perfect ­dictatorship.”

“This felt like the old-style PRI manipulation of trying to go through the motions of doing another study — which was really needed — and using it just to muddy the waters,” said Daniel Wilkinson, managing director for the Americas division of Human Rights Watch, who also attended. “I think the report’s absolutely devastating.”

Among the most serious allegations to arise from the study is the claim of widespread torture by police. A study of 17 of the roughly 110 suspects arrested in the case showed cuts, bruises and other injuries consistent with torture. One suspect reported a plastic bag had been held over his head until he nearly asphyxiated.

“They began beating me again and gave me electric shocks, they put a rag over my nose and poured water on it,” one suspect, Patricio Reyes Landa, said in testimony cited in the report, the Associated Press reported. “They gave me shocks on the inside of my mouth and my testicles. They put a bag over my face so I couldn’t breathe. It went on for hours.”

In their two reports, exceeding 1,000 pages in total, the independent investigators littered the government’s case with holes.

The government’s conclusion, what the former attorney general called the “historic truth,” that the students were burned in the trash dump in the hills above the town of Cocula, has been refuted by the group of experts and an Argentine forensic team that has studied the case, using fire experts to examine the dump site. They did not find evidence that a fire large enough to consume 43 bodies occurred there.

Authorities have said that bone fragments from the one student confirmed by a lab in Innsbruck, Austria, were found in a plastic bag along the Rio San Juan in Cocula, well below the trash dump. But the experts have photos apparently showing that police and prosecutors were at that same site the day before the bag was supposedly found and carried similar bags, raising the prospect that evidence was planted there.

The report found that police, apparently in cahoots with a local drug cartel, set up checkpoints in an area far larger than previously known to prevent the Ayotzinapa students from escaping Iguala on buses late on the night of Sept. 26, 2014. The experts also concluded that the students were not involved in organized crime and were enrolled at the Raul Isidro Burgos Normal school in ­Ayotzinapa.Although authorities have alleged the students’ cellphones were burned the night they were killed, the investigators found that some phones were active for months after the disappearance.

“The group wasn’t able to find the students, obviously,” Wilkinson said. “But it’s shown to what extent this government was willing to go, not to find the students, but to impose a fiction that it calls a historic truth.”