Officials said Davis, of Gainesville, had "commercial sex" with the teen twice after responding to the Backpage.com ads. The girl also claimed that he filmed and took photos during their encounters, the Sun reported.
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Davis admitted to the sex but told authorities that he believed the girl was 19 based on what the ads said, investigators said.
Authorities originally charged Davis with using a minor to produce child pornography and child sex trafficking. The child porn charge was dropped as part of a plea deal, Reuters reported.
Davis' prison sentence will be followed by 25 years of supervised release, the U.S. Attorney's Office said. He also must pay $3,000 in victim restitution and $5,000 for a special assessment, authorities said.
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