MEXICO CITY -- Drug gang members have described a horrific effort to make 43 teachers college students disappear, piling their bodies like cord wood on a pyre that burned for 15 hours and then wading into the ashes to pulverize, bag and dispose of remaining teeth and bones.

In a sombre presentation, Attorney General Jesus Murillo Karam laid out on Friday what investigators think happened to the students who have not been seen since being attacked by police Sept. 26 in the southern city of Iguala.

He played video of purported gang members confessing to the killings and telling what they did. Another video showed hundreds of charred fragments of bone and teeth that had been dumped in and along the San Juan River in the neighbouring town of Cocula.

The attorney general said the state of the remains will make it hard to say definitely whether they are the students.

"The high level of degradation caused by the fire in the remains make it very difficult to extract the DNA that will allow an identification," Murillo Karam said at a news conference.

He said authorities were putting their last hope with a specialized laboratory in Austria. It is not known how long the process could take.

Police officers who took away the students after the confrontation in Iguala allegedly handed the young men over to the Guerreros Unidos cartel. Gang members apparently thought the students were members of a rival gang or believed they had been sent to disrupt a public event held by the wife of the Iguala mayor, who is alleged to have ties with the gang.

Murillo Karam said there is no evidence the students were involved in organized crime.

Some 74 people have been detained so far in the case. Authorities say it started when police, under orders of then Iguala Mayor Jose Luis Abarca, opened fire on students who were in Iguala collecting donations and had commandeered public buses. Six people were killed in two confrontations before the 43 students were taken away and allegedly handed over to Guerreros Unidos.

Abarca and his wife, who were captured Tuesday after weeks of being on the run, are among those in custody.

Parents reacting to Murillo Karam's report said they have lost trust in anything the government says.

"As long as there are no results, our sons are alive," said Felipe de la Cruz, the father of one of the disappeared. "Today they're trying to close the case this way ... a blatant way to further our torture by the federal government."

Until there are identifications, Mexico could remain on edge, as it has since the disappearances six weeks ago in Iguala, which is in Guerrero state. People have held angry marches across Mexico and in other countries, and protesters have sacked or burned public buildings in the Guerrero state capital and also attacked properties owned by Abarca.

According to confessions made by suspects to prosecutors, the killing of the students was on an industrial scale: They were driven to killing grounds in a dump truck so tightly packed about 15 of the young men suffocated to death. The others were then slain, apparently shot to death, and all were put on the fire, which was fueled by gasoline, diesel, wood, plastic and old tires.

The suspects even burned their own clothes to destroy evidence, they said.

At the news conference, Murillo Karam confirmed that human remains found in mass graves discovered soon after the students went missing did not include any of the 43 young men enrolled at a radical rural teachers college. Those graves held women and men believed to have been killed in August, he said.

Among the bodies found in the course of the investigation were a father and son. By searching for reports of father-son disappearances, authorities were able to make a positive identification. Murillo Karam said the victims, whose names he did not reveal, apparently made a call before disappearing to say they were being detained by Iguala police.