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A 52-year-old photographer is facing numerous sex crime allegations for allegedly assaulting dozens of models — some of them underage, the FBI said Wednesday.

The agency described Robert Arnold Koester as a “suspected serial sexual predator,” in a news release announcing 57 charges in California and Oregon.

Koester is believed to have taken nude photos of models and is alleged to have sexually assaulted many of them, including people younger than 18 years old, the FBI said in a statement. He primarily photographed models on the West Coast and also went by the names Bert Kay, Rhake Winter and Qitooly, the FBI said.

He was arrested in November in Carlsbad, California, and is in custody in San Diego, according to the FBI.

He is facing 23 felony charges in San Diego County, including child sexual abuse and production of child pornography. Jonathan Lynn, an attorney at Wallin & Klarich, told CNN that Koester has pleaded not guilty to all of them.

Lynn said he “cannot comment on the charges from Oregon as I am not representing Mr. Koester in Oregon and grand jury proceedings are confidential.”

In Yamhill County, Oregon, where he owns a farm, Koester has been charged with 32 felony counts, including first-degree rape, first-degree sodomy, first-degree unlawful sexual penetration and causing another person to ingest a controlled substance, the FBI said. The Oregon charges involve four separate victims, one of whom was a minor, the FBI said. Koester’s bail on the Yamhill County charges is $2.5 million, according to the FBI. It was not immediately clear if he has an attorney in those cases.

FBI seeking help from public

Koester also has been charged with two federal counts of sexual exploitation of a minor in California, according to the FBI.

Those two charges, detailed in a federal criminal complaint, center on allegations that Koester took nude modeling photos of two underage girls in separate incidents. In each photo session, Koester posed a girl in sexualized positions and touched her all over her body, including in her genital area, the complaint states.

He told investigators that he did so because he wanted to help “prepare her for a future modeling career with Playboy,” according to a complaint. Another victim said that he told her he wanted to photograph her naked for a book he was making, the complaint says.

One of the alleged victims said that Koester told her not to tell anyone about the photos because he did not want to get into trouble because she was a minor, the complaint states.

After one girl and her parents went to police to report him, police obtained a search warrant and Koester was arrested without incident. He admitted that he had taken similar photos with other minors over the past several years, the complaint states.

“Koester argued that he took nude photographs of underage models to get them comfortable with taking pornographic photographs for their adult modeling careers,” the complaint states.

Authorities say they have already identified victims in multiple cities across the United States.

The FBI said Koester could have potentially been committing crimes since 1994. The bureau is asking for the public’s assistance finding other potential victims. Information may be submitted confidentially by email to modelcase@fbi.gov or by filling out an online questionnaire.