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The FBI and other law enforcement agencies are looking for potential victims of a photographer who has been accused of taking nude photos of models and sexually assaulting many of them, possibly going back to 1994.

Robert Arnold Koester, 52, also known by the names Bert Kay, Rhake Winter, and Qitooly, was arrested in Carlsbad, California, in November and faces charges on 23 felony counts, including child sexual abuse and production of child pornography, the FBI said.

The FBI called Koester a "suspected serial sexual predator" and said that he is accused of sexually assaulting models he photographed, including those under the age of 18.

He "has potentially been engaging in these criminal acts starting in 1994" and continuing until his arrest in California in November, the FBI said in a statement Wednesday.

Robert Arnold Koester, a photographer, has been indicted on rape charges after he allegedly brought young women and girls to a rural property, drugged and assaulted them. FBI via AP

Koester was also on Jan. 30 charged by a Yamhill County, Oregon, grand jury with 32 felony counts involving four alleged victims, one of whom is a minor, according to the FBI. The charges include first-degree rape and other sexual offenses, as well as causing another person to ingest a controlled substance, the FBI said.

Koester owns a farm in Yamhill County, the FBI said.

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Yamhill County District Attorney Brad Berry said there are four known victims in Oregon and they suspect there are "many more," according to NBC affiliate KGW of Portland. Berry said the case in Oregon began with Koester's arrest in California.

"He would bring models from other states to Oregon, bring them out to the property where they would be photographed," Berry said at a news conference Wednesday.

"We're looking for information from individuals who may have been victims, may think that they potentially were victims" or from those who might know of people who were at the property, he said, adding that investigators have photographs and other videos related to people who have not yet been identified.

"Alleged victims have been identified in multiple cities across the United States," the FBI said, adding that at the time of his arrest in late 2018 he was working as a professional photographer "engaged in the photography of models, primarily on the West Coast of the United States."

The FBI is also encouraging anyone with information about potential victims, or information about the case, to contact authorities.

Koester is being held in jail in San Diego, according to online jail records. Online federal court records and San Diego court records do not list an attorney for Koester.

In Southern California, he is also charged with two federal counts of sexual exploitation of a minor involving two minors.

A criminal complaint in that case alleges an underage girl and her parents made a report to Carlsbad police on or about Nov. 13 that Koester took nude pictures of her, and Koester was arrested. Koester claimed he took nude photos of underage models to get them comfortable with taking pornographic photographs for their adult modeling careers.

After media reports about the case, another underage girl and her parents made a complaint to Carlsbad police and said he also took nude pictures of her, according to the federal criminal complaint. Some of the photos and videos recovered from Koester's computer depicted sexual acts, with some appearing to show a victim unconscious and non-responsive, the FBI wrote in the complaint.

In the Carlsbad cases, some other women have come forward after Koester’s arrest, NBC San Diego reported in November.

Deputy District Attorney Dan Owens praised one of the girls who came forward, according to the station.

After Koester's arraignment in November, the mother of one of the girls who had complained to police told the station that her daughter is "very brave" and that "it was meant to be that she was able to have the strength to come forward and get the word out."

"We're thankful he's charged and that he's going to be paying for what he's done," the mother of the girl said. "We know justice is going to be served."