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Los Angeles County Sheriff Jim McDonnell is warning parents about the vulnerability of children and young teens who take nude selfies and post them online.

In an open letter to parents, McDonnell wrote that obscene selfies "are forever present on the Internet, viewed and traded like baseball cards by child molesters, predators, and extortionists."

"What may have started as a copycat online trend is now manifesting itself as a nexus between our teens and the predators who seek to exploit them for profit."

William La Jeunesse reported on "Happening Now" that McDonnell said last year alone, his deputies investigated more than 500 cases of boys and girls posting nude photos of themselves. This year, they have already investigated more than 80 cases.

La Jeunesse said that McDonnell is urging parents to speak to their children about how the photos they post on the Internet will exist forever.

He said that part of the problem is that parents assume their child isn't involved in anything like this, so they never have the discussion.

"It sounds like common sense, of course, but as we know a lot of 12-year-olds - and adults - don't have common sense," La Jeunesse said. 

Watch La Jeunesse's report above.

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