SPOKANE, Wash. -

A local father is speaking out about what he calls his worst nightmare, watching his daughter spiral out of control. He says it seemed overnight she began failing classes and acting out. But, he had no idea she was the victim of sexual abuse.

Detectives say the 11-year-old girl was the victim of 85-year-old Marvin Petersen. He was arrested last week for child rape and other charges. Detectives believe he's been sexually abusing children for decades.

"Detectives uncovered information that this suspect's behavior has possibly been going on for 40, 50, 60 years and dating all the way back to Oregon," said Mark Gregory with Spokane County Sheriff's Office.

The father of the girl in this most recent case says he's not surprised to hear that.

"The way he groomed her is, basically, it was like a job for him. He's a professional at it," he said.

The father is not being identified to protect the identity of his family. He says he noticed a dramatic shift in his daughter's behavior about a year ago.

"11-year-old girl spiraled out of control, no reason why," he said. "One week she's got A's, B's and C's and basically the next week she's running away, wanting to kill herself. It's been an absolute nightmare."

Experts say those are some common warning signs for children who have been victimized by a sexual abuser. Luther Community Services lists several warning signs for parents to look out for.

"They could be more withdrawn, or they could be more acting out and angry," said Dan Fox, a clinical manager at the facility. "There's sleeping pattern changes, maybe they're no longer doing well in school, or maybe they're having problems with friendships they never had problems with before."