Investigators in Mexico say they are now certain that 43 college students missing since September were killed and incinerated after they were seized by police in southern Guerrero state.
It was the first time Mexican attorney general Jesus Murillo Karam said definitely that all were dead, even though Mexican authorities have DNA identification for only one student.
A laboratory in Innsbruck, Austria, which examined DNA samples believed to be linked to the case, declared it was impossible to identify the others.
The attorney general on Tuesday cited confessions and forensic evidence from an area near a garbage dump where the September 26 crime occurred that showed the fuel and temperature of the fire were sufficient to turn 43 bodies into ashes.
"The evidence allows us to determine that the students were kidnapped, killed, burned and thrown into the river," Karam said in a news conference that included a video reconstruction of the mass slaying and of the investigation into the case.
He added that "there is not a single shred of evidence that the army intervened ... not a single shred of evidence of the participation of the army," as relatives of the victims have claimed.
Karam said the motive was that the members of a local gang, the Guerreros Unidos, mistakenly believed the young men were rival gang members when they hijacked some public transit buses in Iguala.
The links between the police who allegedly seized the group and the local gang members remain unclear but many of the suspects testified that they knew the men were students.
The students, known for commandeering buses and taking over toll booths to support their leftist causes, said they were taking the buses for transport to an upcoming demonstration in Mexico City.
Karam's explanation seemed unlikely to quell the controversy and doubts about the case, in which the federal government has been criticised for acting slowly and callously.
Thousands of people demonstrated in Mexico City Monday night, demanding the students be returned alive.
"They pretty much gave the same story as they had given two months ago. There are not many additional details," said analyst Alejandro Hope. "They are searching for closure but I'm not sure they're going to get it."
Not 'sufficient evidence'
The attorney general has come under attack from many quarters, including the students' relatives and fire experts, who say the government's version of what happened is implausible. Family members are still searching in hopes of finding the students alive.
The Argentine Forensic Anthropologists, an independent team hired by parents to work with federal investigators, told The Associated Press on Sunday that there is still not "sufficient evidence" to link the charred remains found by authorities in a river in the town of Cocula to what happened at the garbage dump.
Karam said the information was based as well on 386 declarations, 487 forensic tests, 16 raids and two reconstructions.
So far 99 people have been detained in connection with the crime, including the former mayor of Iguala, Jose Luis Abarca.
The case has sparked protests inside and outside Mexico over the four months since the students disappeared, and has forced the Mexican government to turn its attention from touting economic and education reforms to dealing with the country's crime and insecurity problems.
Analysts said protests were likely to continue as long as there was no unimpeachable evidence that the remains belonged to the students.