Mexican officials announced the arrest of four gang members thought to be operatives in the disappearance of a group of students who went missing more than a month ago in the southern state of Guerrero. Authorities said the arrests led them to the discovery of a new mass grave that may contain the remains of the missing youths.

Mexico’s attorney general, Jesús Murillo Karam said Monday evening that police arrested four members of the Guerreros Unidos gang who were responsible for the disappearances of 43 students from the Ayotzinapa teachers’ training college on Sept. 26. The gang members’ statements led federal authorities to find the mass grave in Cocula, a town about 10 miles away from Iguala, where the students were last seen.

“Today we now have those who organized the disappearances of these youths,” Murillo said at a press conference Monday night. He said that one detainee told police that he and another received a large group of people on the night the students went missing, while the other two served as lookouts. Murillo said investigators were working on corroborating their stories. Meanwhile, authorities are beginning DNA testing on the bodies found in the Cocula mass grave.

Around 50 people in total have now been arrested in connection with the missing students, who were last seen after Iguala police shot at them while they were taking buses back to their hometown. Authorities believe that Iguala’s mayor, widely thought to have deep links to organized crime groups, ordered the police to go after the students and turn them over to Guerreros Unidos members for fear the students would disrupt a speech being given by his wife that same day.

The disappearances of the students, all young men, have roiled Mexico and set off mass protests against corruption and impunity within Mexican police forces. Earlier this month, officials uncovered several mass graves on the outskirts of Iguala, but the bodies did not turn out to match any of the missing men from the Ayotzinapa school.