Attorney Vidulfo Rosales told The Associated Press that officials also had not determined how many bodies are in the six burial pits discovered on a hillside on the outskirts of Iguala, a city in southern Mexico where a series of violent incidents last weekend resulted in six shooting deaths and more than two dozen people injured.

Rosales said relatives of 37 of the missing 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 Guerrero 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.

Guerrero State Prosecutor Inaky Blanco said Saturday evening that eight more people had been arrested in the case, including seven members of a crime gang, and said some of those newly detained had provided key clues that led to the clandestine grave site.

Officials said previously that 22 Iguala city police officers were detained soon after last weekend's violence.

State prosecutors have said the troubles began after students hijacked buses for a protest. City police opened fire on the buses, killing three youths and wounding 25, prosecutors said. A few hours later, unidentified masked gunmen shot at two taxis and a bus carrying a soccer team on the main highway, killing two people on the bus and one in a taxi. (***)

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