Officers did not offer a motive for the attack.
Police would not explain the violent threat in detail but said it was made on social media and posted Thursday.
A San Bernardino County probation officer has been arrested on suspicion of having a sexual relationship with a minor.
California is a national leader in reducing the number of juvenile offenders incarcerated in state and county lockups, according to a study advocacy groups released Tuesday.
A standing room only crowd of parents and supporters attended a school board meeting in Fontana Wednesday and most said they were there to support a teacher who is accused of taking part in a hazing incident.
Authorities are investigating how two teens in custody for violent crimes were found having sex while in custody at Juvenile Hall.
Stephen Wesley was accused of matching up six students from rival gangs and videotaping a fight, according to prosecutors.