ALBUQUERQUE, N.M. (AP) — A fourth officer has resigned during an investigation into alleged wrongdoing by officers assigned to a unit charged with stopping impaired drivers, the police department in Albuquerque, New Mexico, said.

Investigators had been trying to schedule an interview with Nelson Ortiz, but he resigned on Thursday, the department said in a news release.

Ortiz joined the department in January 2016 was assigned to the DWI unit from 2018 to 2021.

The police department launched the probe into officers who were currently or previously working for the unit. The FBI is also investigating.

Five officers were placed on administrative leave during the investigation. The three officers who resigned previously were Justin Hunt, Honorio Alba and Harvey Johnson.

According to documents obtained by the Albuquerque Journal, the federal probe began after a stop by an officer in August in which he allegedly told the driver to contact a specific attorney to ensure that a case would not be filed.

The investigation has partly focused on DWI criminal cases filed by specific officers that were eventually dismissed, the Journal reported. More than 150 cases alleging that motorists drove while intoxicated have been dismissed as part of the probe.

Three Albuquerque police officers combined filed 136 of the 152 DWI cases, and at least 107 of those were filed last year, which was 10% of such cases for the department that year.