Mexico City (CNN) -- As Mexico City authorities unravel the workings behind the mass kidnapping and killing of 13 bar-goers, some unsettling developments have surfaced. Four of those arrested in connection with the crimes are police officers.
Corruption within law enforcement is not uncommon, but the alleged involvement of police officers in this sizable plot, which drew international attention, is a stark reminder of the challenges facing Mexico.
Mexico City prosecutors on Tuesday announced the arrest of the fourth officer: Daniel Valverde Trujillo, 35. He is accused of kidnapping and is being detained temporarily while the allegations against him are investigated, the Mexico City Attorney General's Office said.
The 13 youths disappeared on May 26 from an after-hours bar named Heaven in the capital's Zona Rosa district. In August, their bodies were found in a mass grave near Mexico City.
On September 19, prosecutors announced the arrest of Edgar Ernesto Gutierrez Vera, a patrolman in the Zona Rosa area. He is accused of "active and direct participation" in the mass kidnapping and killing.
At least 14 others have been arrested as the investigation moves forward. Prosecutors say some of those initial suspects alleged that Gutierrez Vera provided protection for the leader of the gang implicated in the mass kidnapping.
Two other arrests were announced Monday. Police officers Jorge Clemente Martinez Perez and Jose Zequera Zavala, who also patrol the Zona Rosa sector, are linked to the crimes, prosecutors said.
The four officers have not been formally charged are detained provisionally while investigators determine if there is enough evidence to bring charges.
Mexico City Prosecutor Rodolfo Rios Garza said last month that the motive for the killings was revenge for a slaying that occurred in another bar in the capital.
Victims' family members have said they don't trust Mexican authorities and have called for an independent international investigation of the case.
CNN's Mariano Castillo contributed to this report.