Suspects in the disappearance of 43 college students have described a macabre and complicated mass murder and incineration of the victims carried out over an entire day and ending with their ashen remains being dumped into a river, Mexican authorities said Friday.

In a sombre, lengthy explanation of the investigation, Attorney General Jesus Murillo Karam played video showing hundreds of charred fragments of bone and teeth fished from the river and its banks. He said it will be very difficult to extract DNA to confirm that they are the students missing since Sept. 26 after an attack by police in the southern state of Guerrero.

"I know the enormous pain the information we've obtained causes the family members, a pain we all share," Murillo Karam said at a news conference. "The statements and information that we have gotten unfortunately points to the murder of a large number of people in the municipality of Cocula."

Some 74 people have been detained so far in a case that prosecutors have said started when police, under orders of the mayor and working with a drug gang, opened fire on students in the city of Iguala, where they were collecting donations and had commandeered public buses. Six people were killed in two confrontations before the 43 were taken away and handed over to members of the Guerreros Unidos cartel.

Murillo Karam said authorities are searching for more suspects.

Mexico Violence

A demonstrators with her mouth taped shut and the number 43 painted on her face marched in protest for the disappearance of 43 students in Mexico earlier this week. Prosecutors now say they believe they have found the charred remains of the missing students. (Marco Ugarte/Associated Press)

"As long as there are no results, our sons are alive," Felipe de la Cruz, the father of one of the disappeared, said in a statement criticizing the federal government, who many have faulted for its false starts and slow response. "Today they're trying to close the case this way ... a blatant way to further our torture by the federal government."

In the most comprehensive accounting to date of the disappearances and the subsequent investigation, Murillo Karam showed videotaped confessions by those who testified to loading the students in dump trucks and carrying them to a landfill site in Cocula, a city near Iguala. Some 15 of the students were already dead when they arrived at the site and the rest were shot, according to the suspects.

They then built an enormous funeral pyre that burned from midnight until 2 or 3 p.m. along the River San Juan in Cocula. "They assigned guards in shifts to make sure the fire lasted for hours, throwing diesel, gasoline, tires, wood and plastic," Murillo Karam said.

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

No DNA confirmation 

It was about 5:30 p.m. when the ashes had cooled enough to be handled. Those who disposed of the bodies were told to break up the burned bones, place them in black plastic garbage bags and empty them into the river.

Murillo Karam said the teeth were so badly charred that they practically dissolved into dust at the touch.

Mexico students

Mexico's Attorney General Jesus Murillo Karam says it will be challenging to get the DNA required to confirm the remains are those of the missing students. (Tomas Bravo/Reuters)

"The high level of degradation caused by the fire in the remains we found make it very difficult to extract the DNA that will allow an identification," he said.

Murillo Karma had told relatives of the missing students earlier Friday that authorities believe their children are these charred remains, but have no DNA confirmation.

"The meeting with the attorney general was tense, because we don't believe them anymore," said Manuel Martinez, a spokesman for the families and guardian of two of the missing young men.

Murillo Karam also confirmed at the news conference that human remains found in mass graves discovered 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 use, apparently made a call before disappearing to say they were being detained by Iguala police.

"To the parents of missing youths, we tell them we will not relent until justice is done," said President Enrique Pena Nieto. "The events in Iguala have outraged and shocked us all and have aroused Mexican solidarity with the young normalistas."

Authorities say Iguala's mayor sent police to intercept the students, who came to town to collect money and had commandeered buses. Officers opened fire, killing six people, and prosecutors say the police then handed the 43 students over to a drug gang.

Those arrested in the case include Iguala Mayor Jose Luis Abarca and his wife, Maria de los Angeles Pinesa, who were found hiding Tuesday in a rough Mexico City neighborhood.

Relatives of many of the missing students have been camped at their school, the Rural Normal School of Ayotzinapa, since the days immediately following their disappearance from Iguala.

Mexico students

Protesters had been demanding the safe return of the 43 missing students. (Ronaldo Schemidt/AFP/Getty Images)