A 31-year-old man was sentenced Monday to more than 24 years in federal prison for posing online as a teenage boy dying of leukemia in an effort to coerce young girls into sending him sexually explicit images.

Joshua Kistler chatted regularly with at least nine girls nationwide who ranged in age from 12 to 14. He used the condition to gain the girls' sympathy, according to the U.S. Department of Justice.

He even sent them pictures of his stepson, an actual 15-year old, to further his story, authorities said.

He also posed as the stepfather of his online persona. In one case, a victim and her mother visited the "stepfather" during a family trip, believing the teenage boy was away receiving treatment.

Several victims said they loved and trusted Kistler, who asked them to send photos of them nude or perform sexual acts for him in front of Web cameras.

Kistler, who pleaded guilty to charges of producing child pornography, apologized during his appearance in U.S. District Court.

"I'm sorry for the harm I've caused," he said. "I wasn't aware I was causing any harm."

The victim whose case led to Kistler's arrest was a Utah girl who started chatting online with him when she was 12.

According to the Justice Department, Kistler told the victim that he had cut off contact with another girl because of the Utah girl's jealousy and that the other girl had killed herself. Overcome with guilt, the Utah girl attempted suicide, and law enforcers became involved.

The girl attempted suicide a second time after the mother tried to stop the relationship.

The mother read the words of her daughter in court.

"He taught me stuff a 12-year old should not know," KGW-TV reported. "My love of my life turned out to be a 30-year-old pedophile."

Kistler told agents that he knew what he was doing was wrong and that he had always "been a collector."