Authorities are investigating whether several bodies found in clandestine graves in southern Mexico are those of 43 students who disappeared after a deadly police shooting last week.
The pits were found Saturday on a hill in a community outside Iguala, the town where the students were last seen and where witnesses say municipal police officers whisked several of them away.
Inaky Blanco, chief prosecutor for the violence-plagued state of Guerrero, declined to say how many bodies were buried in the pits outside Iguala, 200 kilometers (125 miles) south of Mexico City.
"We still can't talk about an exact number of bodies. We are still working at the site," Blanco told a news conference in the state capital, Chilpancingo.
But two police officers at the scene in the community of Pueblo Viejo told AFP that at least 15 bodies were exhumed from the site, which was cordoned off and guarded by scores of troops and police.
Juan Lopez Villanueva, an official from the National Human Rights Commission, said that six pits were found up a steep hill probably inaccessible by car.
Four forensic services vans left for the morgue late Thursday carrying bodies in silver bags. Authorities are conducting DNA analysis to identify the victims.
The graves were found after some of the 30 suspects detained in the case told authorities about their location, Blanco said. The detainees include 22 police officers and gang members.
If the students are in those pits, it would be one of the worst slaughters that Mexico has witnessed since the drug war intensified in 2006, leaving 80,000 people dead to date.
The students from a teacher training college disappeared last weekend after Iguala police officers shot at buses that the group had seized to return home after holding fundraising activities on September 26. Three students were killed.
Another three people died when police and suspected gang members shot at another bus carrying football players on the outskirts of town.
A survivor said in an interview that the officers took away 30 to 40 students in patrol cars.
Blanco said investigators had confirmed suspicions that a criminal organization, the Guerreros Unidos, was involved in last week's crimes and that local police officers belong to the gang.
Authorities have issue an arrest warrant for Iguala's mayor, who has fled.
Governor Angel Aguirre appealed for calm in his state, which is mired in poverty, gang violence and social unrest.
"I call on all (Guerrero state residents) to maintain harmony, non-confrontation, and avoid violence," he said, offering his support to the families of those who were "savagely massacred."
The missing students are from a teacher training college near Chilpancingo known as a hotbed of protests.
Thousands of students and teachers blocked the highway between Chilpancingo and Acapulco for hours on Thursday, demanding help from federal authorities to find the missing.
The police's links to organized crime has raised fears about the fate of the students in a country where drug cartels regularly hide bodies in mass graves.
Around 30 bodies were found in mass graves in Iguala alone this year.
"We are very worried. The families are very anxious," said Vidulfo Rosales, a human rights lawyer representing relatives of the missing.
The United Nations has called the case "one of the most terrible events of recent times."