IGUALA, Mexico -- A clandestine grave site with multiple burial pits was found outside a town where violence last weekend resulted in six deaths and the disappearance of 43 students after a clash with police, Mexican officials said Saturday.

Guerrero State Prosecutor Inaky Blanco said the grave site was on the outskirts of Iguala, a town about 120 miles (200 kilometres) south of Mexico City. He did not give say how many bodies were in the graves or whether there was any indication that the remains could be the missing students.

Juan Lopez Villanueva, an official with the Mexican government's National Human Rights Commission, said later that a total of six burial pits had been discovered. He also did not comment on whether the remains could be the students.

The clandestine grave was on a hillside in rugged territory of Iguala's poor Pueblo Viejo district, and was heavily guarded by soldiers, marines and federal and state police who kept journalists away from the site. A helicopter landed inside the cordoned-off area at midafternoon.

Iguala was rocked by a series of clashes and shootings late Sept. 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 had wounds.

A few hours later, unidentified masked men fired shots 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.

After the violence, Guerrero state authorities said 57 students had been reported missing since the protesters' confrontation with police. That number was later reduced to 43.

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.

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

Vidulfo Rosales, a lawyer for a local human rights group who is assisting the families of the missing students, said before the burial pit was 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 trucks.

"The suspicion, the hypothesis, is that they are being held by organized crime gangs that operated in collusion with the police," Rosales said.

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

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

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. Two state police officers were charged in the shootings.

During that confrontation, students apparently set fire to pumps at a gas station on the highway when federal and state police moved in to quell the protest, and a gas station employee later died of burns suffered in the attack.