PHOENIX — The daughter of a county supervisor has been arrested on suspicion of sexual misconduct with the same teenage boy that her mother is accused of sexually abusing over a three-year period, police said yesterday.

Rachel Katherine Brock, 21, was arrested Wednesday on three counts of sexual conduct with a minor and one count of transmitting obscene material as part of an ongoing investigation surrounding her mother, 48-year-old Susan Brock.

Both women were being held without bond at the Maricopa County jail.

Rachel Brock is the daughter of Maricopa County Supervisor Fulton Brock, and Susan Brock is his wife of 28 years. The family lived together in the Phoenix suburb of Chandler but Fulton Brock filed for divorce after his wife’s arrest.

Rachel Brock is accused of committing numerous sex acts with the teenage boy between February 2007 and August 2008, and sending him nude photos and an explicit video of herself; none of the acts involved intercourse. The boy was 14 at the time, and Rachel Brock was 18, classifying the crimes as dangerous crimes against children.

Police said that between August 2007 and October of this year, the teen met Susan Brock for sexual trysts. Susan Brock reportedly provided the boy with cellphones, and police seized text messages reportedly recording sexual exchanges between the two.

Chandler police Sergeant Joe Favazzo said it appears that Susan and Rachel Brock didn’t know about each other’s relationship with the teen.

The boy, now 17, told police that Rachel Brock began abusing him in 2007 by inappropriately touching him during a trip to California, and it escalated to other sexual contact in a vehicle about a month later, according to a court document released yesterday. The boy also told police there was sexual contact with Rachel Brock at the Brocks’ home and at a property owned by the Brock family.

Susan Brock was arrested in October on two counts of child molestation and two counts of sexual contact with a minor involving the same boy.

The teen told police Susan Brock would pick him up at school or home and drive him to secluded areas where they would have sexual contact in her car, although there was no intercourse, according to a court document.

Authorities say Susan Brock also helped the boy meet his girlfriend and provide places where the young couple could have sex.

In a statement yesterday, Fulton Brock said “shocked and devastated are not sufficient words to describe the news this day or what has transpired over the last two months.’’

“I have filed to divorce my wife. I cannot divorce my daughter,’’ Fulton Brock said. “She is my blood. I will always be her dad. And she needs me now more than ever.’’

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