LOS ANGELES (CBSLA.com) — Nude selfies are viewed and traded like baseball cards among child molesters, predators, and extortionists, the Los Angeles County sheriff warned in an open letter Sunday.

Addressing the nearly 700-word letter to parents, Jim McDonnell specifically charged that nude selfies are forever present on the Internet.

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“All too often,” he wrote, “these images end up on the Internet or in the hands of child predators, some of whom actually make contact with these children with specific intent of luring them into a relationship.”

In other instances, he said, the children are extorted for additional pictures or video and even money.

“These cases slice across all socio-economic and racial lines,” he cautioned.

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Officials at the Los Angeles County Sheriff Department said its Human Trafficking Bureau in 2015 investigated more than 500 cases of nude photos of boys and girls as young as 8.

“Of these cases, one in four photos involved young teenage girls and boys who had taken nude ‘selfies,’ perhaps as an ‘act of love’ for a boyfriend or girlfriend, an act of teenage rebellion, a cry for attention, or did so because they were duped,” he said.

McDonnell said in just the previous 2 ½ months, detectives have investigated 81 cases involving nude or compromising photos and video of children and teens on the net.

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Directing a child to make, send, or possess these photos, McDonnell wrote, is against both state and federal law.