A clandestine grave site with multiple burial pits holding an undetermined number of bodies has been found outside a town in Mexico's Guerrero state where violence last week resulted in six deaths and the disappearance of 43 students.

Inaky Blanco, Guerrero's prosecutor, said on Saturday the grave site was on the outskirts of Iguala, about 200km south of Mexico City.

He said the remains would be sent to Mexico's forensic service to determine whether or not the corpses match those of the missing students.

"In the next few hours we will determine the cause of death and the number of bodies," a spokesman for the state attorney general's office said.

While there was no official count, Reuters reported that the remains of up to 20 people were found.

A member of the Mexican government's National Human Rights Commission told Al Jazeera that he believed the remains, found in six burial pits, were indeed of the missing students. He said the bodies were burned and looked as if they were put into the pits recently.

The graves were found on a hillside in rugged territory of Iguala's poor Pueblo Viejo district, and was heavily guarded by dozens of soldiers, marines and federal and state police who kept journalists away from the site.

Series of shootings

Violence is frequent in Guerrero, a southern state where poverty feeds social unrest and drug gangs clash over territory.

Iguala was rocked by a series of clashes and shootings late on September 27 and early the next day.

State prosecutors have said the first bloodshed occurred when city police shot at buses that had been hijacked by protesting students from a teachers college.

Three youths were killed and 25 people were wounded.

A few hours later, unidentified masked men fired shots at two taxis and a bus carrying a football team on the main highway, killing two people on the bus and one in a taxi.

After the violence, Guerrero state authorities said 43 students had been reported missing since the protesters' confrontation with police.

Blanco has said local police are being investigated for roles in the disappearance.

He said state investigators had obtained videos showing that local police arrested an undetermined number of students after the first incident and took them away.

Officials say 22 officers are facing homicide charges, according to the Associated Press.

Angel Aguirre, Guerrero's governor, has said investigators are also looking at possible involvement of organised crime groups, which he charged have infiltrated the town government.

Mayor suspected

Vidulfo Rosales, a lawyer for a local human rights group who is assisting the families of the missing students, said before the burial pits were found that relatives believed the youths had been turned over to a drug gang by police.

Rosales said some students who escaped the shooting said they saw other students being carried away in several police pickup vehicles.

Al Jazeera's Adam Raney, reporting from the area where the graves were found, said parents of the students had been pushing officials to get more involved in the case.

"The fact is it took several days just for the governor to speak to them and when he did, he just blamed the mayor of Iguala who has now fled that town. He's under indictment and under suspicion of taking part somehow in the abduction and disappearance of those 43 students."

The Aytozinapa Normal school attended by the missing students, like many other schools in Mexico's "rural teachers college" system, is known for radical protests that often involve hijacking buses and delivery lorries.

In December 2011, two students from Aytozinapa died in a clash with police on the highway that leads to the Pacific coast resort of Acapulco.

Students had allegedly hijacked buses and blocked the road to press demands for more funding and assured jobs after graduation.