Mexican authorities Sunday intensified efforts to find 43 students who disappeared more than three weeks ago, using dogs, horses and divers to broaden the search.
About 50 vehicles, carrying some 200 members of a special division of the federal police, arrived in Xonacatla, which is located close to the town of Iguala, where the students disappeared on September 26.
"We are scouring all places that could be of interest," said Manelich Castilla, commissioner for the police division, known as the Gendarmeria Nacional, which is leading the operation.
Authorities said the search had been expanded to include bodies of water such as lakes and ponds, as well as in the numerous mines and caves of the mountains around Iguala.
More than 1,200 security forces are now looking for the college students around Iguala, a town of 140,000 inhabitants.
Authorities say Iguala's police force shot at buses carrying the students on September 26 and handed them over to officers in the neighbouring town of Cocula, who then delivered them to the Guerreros Unidos drug gang.
Six people died and 25 were wounded that night.
Investigators are analysing the contents of three mass graves found near Iguala after declaring that 28 bodies in one pit did not belong to the students.
President Enrique Pena Nieto has been under pressure at home and abroad to solve the case in the violence-plagued southern state of Guerrero. Hundreds of students have protested in front of the attorney general's office in Mexico City.
Mexican authorities announced Friday the arrest of the "maximum leader" of a drug gang accused of colluding with crooked police in the disappearance of the students.
A total of 36 municipal officers have been arrested in the case.