MEXICO CITY (AP) — Forty-three missing college students are believed to have been murdered and burned near a municipal garbage dump in the southern state of Guerrero and their remains thrown into a river, Mexico's chief prosecutor said Friday.

In a somber, 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 identities of the victims of a horrific mass murder and incineration that lasted 14 hours.

"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."

"International experts have said they can't give a time frame for when they will get results," he added.

Some 74 people have been detained so far in a case that prosecutors have said started when police attacked student protesters Sept. 26 in the city of Iguala, killing six people and taking away 43 students. Murillo Karam said authorities are searching for more suspects.

In the most comprehensive accounting to date of the disappearances and the subsequent investigation, Murillo Karam showed videotaped confessions by those who allegedly killed the students and built an enormous funeral pyre of tires, wood and fuel along the River San Juan in Cocula, a town near Iguala.

He also confirmed that human remains found in clandestine graves discovered after the students went missing did not include any of the 43 young men. Those graves held women and men believed to have been killed in August, he said.

Mexican authorities told relatives of the missing college students earlier Friday that they had found six bags of unidentified human remains on the river bank. Murillo Karam told families that authorities could not say yet whether the remains were of the students, but the find opened a new avenue of investigation, said Manuel Martinez, a spokesman for the families.

"The meeting with the attorney general was tense, because we don't believe them anymore," said Martinez, who was guardian of two of the missing young men.

The students of a rural teachers college haven't been seen since the confrontation with police in Iguala, which is 80 miles (130 kilometers) southwest of Mexico City. 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.

Detainees include former 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.