The Mexican government Monday offered a reward of 1.5 million pesos ($111,000) for information on the whereabouts or captors of 43 university students missing for nearly a month. The announcement came as Mexican federal police took control of 13 towns in southern Mexico as part of stepped-up efforts to find the students, whose disappearances sparked widespread anger and protests throughout the country.

Pressure has been mounting on the government of President Enrique Pena Nieto to locate the students, who disappeared after a police shootout in Mexico’s Guerrero state Sept. 26. Authorities have accused members of the local police force of having links to organized crime groups, and many allege police collusion with local gang Guerreros Unidos is behind the student disappearances. The mayor and police chief of Iguala, the town where the confrontation between the students and police occurred, are also thought to have organized crime links, and have both gone into hiding.

Security forces discovered multiple mass graves near the town of Iguala, but so far DNA testing has not matched any of the bodies with the missing students. Last week, Mexican officials announced they had arrested Sidronio Casarrubias Salgado, a top leader of the Guerreros Unidos gang. Last week the head of the gang, Benjamin Mondragon Pereda, killed himself during a standoff with federal police. But authorities said it was not clear if Mondragon had any involvement with the student disappearances.  

Meanwhile, the ongoing mystery of the students’ whereabouts has triggered angry protests across the country. Thousands of demonstrators have marched to demand information on the location of the students and an end to corruption in Mexico’s local police forces. Last week, hundreds of protesters overtook Guerrero’s state government buildings and set parts of the complex ablaze. Protesters also overtook town halls in three Guerrero municipalities.