The Modoc County Sheriff's Department said the former Alturas police chief may have given false testimony at the trial of Cherie Rhoades.
Three years ago, Rhoades opened fire inside the Cedarville Rancheria tribal office, killing four people, including three family members.
Her trial was held in Placer County, and in January, a jury handed her the death penalty.
Modoc County says they are legally obligated to disclose that Police Chief Ken Barnes may have given false testimony at her trial to the Placer County District Attorney.
Rhoades' attorney has since filed a motion for a retrial.