IGUALA, Mexico (AP) ― Mexican forensic experts recovered 28 charred bodies from a clandestine grave on the outskirts of this city where police engaged in a deadly clash with student protesters a week ago, Guerrero state’s chief prosecutor said Sunday.

State Prosecutor Inaky Blanco said the corpses were too badly damaged for immediate identification and he could not say whether any of the dead could be some of the 43 college students reported missing after the confrontation with police. He said genetic testing of the remains could take two weeks to two months.

Blanco said one of the 30 people detained in the case had told investigators that 17 students were taken to the grave site and killed there. But he stressed that investigators had not confirmed the person’s story. 

State police and prosecutors have been investigating the Iguala city police for misconduct during a series of violent incidents last weekend that resulted in six shooting deaths and more than two dozen people injured.

Investigators said video showed police taking away an undetermined number of student protesters after a confrontation.

Twenty-two officers were detained soon after the violence, and Blanco has said eight other people were arrested in recent days, including seven members of an organized crime gang.

Blanco said Saturday that some of those arrested had provided key clues that led investigators to the unmarked burial pits on an isolated hillside on the edge of Iguala. He said Sunday that the site in rugged terrain is about 2 kilometers from the nearest road.

The prosecutor said the bodies had been put in pits that were covered over with vegetation that was set on fire after being doused with a flammable substance such as gasoline.

Vidulfo Rosales, a lawyer helping families of the missing students, said relatives of 37 of the young people already had provided DNA samples that will be used to determine if the recovered remains belong to any of the students.

As investigators worked at the grave site, up to 2,000 protesters blocked a main highway in the state capital of Chilpancingo demanding justice. “You took them alive, we want them returned alive,” read a huge banner hung across the road linking Mexico City and Acapulco.

Jesus Lopez, an Acapulco street vendor whose 19-year-old son Giovani is among the missing, said he hoped the remains weren‘t those of the students.

Other relatives “told us that (the remains) were burned, and that they couldn’t be the kids,” Lopez said. “But we’re really nervous.”

Mexico’s National Human Rights Commission opened its own investigation into the case for possible “serious human rights abuses,” such as extrajudicial executions and forced disappearances by Iguala city police.

The commission said in a statement Sunday that it had warned about the “delicate” situation in Guerrero, a southern state where poverty feeds social unrest and drug gangs clash over territory.

Anger over the discovery of the graves exploded Saturday night when a group of young people from the Aytozinapa teachers college attended by the missing protested outside the governor’s residence in Chilpancingo.

They threw Molotov cocktails and overturned a car after state officials told them they would not be allowed to travel to the graves to determine if the bodies are those of their missing classmates.

Like many other schools in Mexico’s “rural teachers college” system, Aytozinapa is known for militant and radical protests.