Mexican officials have arrested four cartel members they suspect are involved in the kidnapping of 43 students who have been missing since September and are feared to be dead.

The arrests came right as another mass grave was discovered near the town of Iguala, where the students were last seen, according to a report from Reuters. Authorities have found other mass graves near Iguala recently but haven't found remains of the missing students.

The students have been missing since they protested and clashed with police on Sept. 26. Mexican authorities have already arrested some members of Iguala's security force for colluding with the Guerreros Unidos gang in the disappearance.

Mexican Attorney General Jesus Murillo said the four Guerreros Unidos gang members in custody were directly involved in the kidnapping. In total, Mexican officials have detained 56 people in connection with the missing students.

"Today we now have those who organized the disappearance of these youths," Murillo said but made no mention of the new mass grave found.

Prior to Monday's discovery of the latest grave, investigators have found 11 other mass graves containing the remains of 38 people in and around Iguala, but none of the remains are of the missing students.

Iguala's mayor and his wife are implicated in the kidnapping, and the two currently are on the lam.

Guerrero's governor resigned over the disappearance, which has sparked protests from a disgruntled public across Mexico's south, including the burning of Iguala's city hall. Replacing former Guerrero Gov. Angel Aguirre, who resigned over strong opposition of his handling of the situation, is Rogelio Ortega Martinez, who was named interim governor on Sunday.

Ortega is a sociologist and was previously the secretary general of Guerrero's public university. He was formerly a social activist and is the son of a rural schoolteacher. He has close ties to the state's ruling Democratic Revolution Party.