Officials are working to determine whether 28 bodies found in a clandestine grave are students who were attacked by police suspected of drug gang links in Mexico.
The bodies are badly damaged and genetic testing could take two weeks to two months, Guerrero State Prosecutor Inaky Blanco said.
It aims to determine if they are some of the 43 college students reported missing after the violent confrontation in Iguala, about 120 miles (200 kilometres) south of Mexico City.
Mr Blanco said one of the people detained in the case had told investigators that 17 students were taken to the grave site on the outskirts of the Iguala and killed.
He said yesterday that as long as the identity of the bodies "has not been resolved we will continue the search" for the missing students.
As investigators worked at the 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.
The bodies had been put in the pits on top of branches and tree trunks, which were doused with a flammable substance and set on fire.
The grave covering and burning appeared fresh, according to one official close to the case. But there are layers of bodies separated by tarpaulins, the official added, indicating that some could have been there for some time.
State police and prosecutors have been investigating the Iguala city police over a series of violent incidents last weekend that resulted in six shooting deaths and left more than two dozen people injured.
Investigators said video showed police taking away an undetermined number of students, who had gone to the city to solicit donations.
Authorities have presented charges against 29 people in the case, including 22 police officers detained soon after the violence. Three of the suspects are fugitives, including Iguala's police chief.
Mr Blanco said they are still investigating the motive for the crime, adding that some of the police have connections to a local drug cartel.
President Enrique Pena Nieto is scheduled to give a statement amid rising international concern over two possible cases of summary executions by Mexican authorities.
An army unit is also under investigation and three soldiers are charged with murder over a June 30 confrontation that killed 22 in neighbouring Mexico state.
Mr Pena Nieto has downplayed violence in the country while focusing on political and economic reforms. His administration reports a drop in homicide, though crimes such as extortion and kidnapping have dramatically increased.
In the Iguala case, Mr Blanco said on Saturday that some of those arrested provided key clues that led investigators to the six unmarked burial pits on an isolated hillside about a mile (two kilometres) from the nearest road.
Vidulfo Rosales, a lawyer helping families of the missing students, said relatives of 37 of the young people had already provided DNA samples that will be used to determine if the recovered remains belong to any of the students.
Like many other schools in Mexico's rural teachers college system, Aytozinapa is known for militant and radical protests.
The mother of one of the missing youths said her son, 17-year-old Luis Angel Abarca Carrillo, had enrolled in the Aytozinapa teachers college attended by the missing students in order to get ahead in life and not be a poor farmer like his brothers.
"But now look what they did to him, he hasn't reappeared," said 60-year-old Margarita Carrillo.
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 it had warned about the "delicate" situation in Guerrero, a southern state where poverty feeds social unrest and drug gangs clash over territory.