FORT WORTH (AP) – A 20-year-old sex offender is returning to prison for fatally injuring the 2-year-old son of his fiancee last year.

Ryan Durham was initially charged with capital murder in the death of Robert Philbin but reached a deal with prosecutors. He received a 40-year prison term in exchange for pleading guilty to the lesser charge of serious bodily injury to a child.

Durham had been caring for the child while the mother and stepgrandfather were working. He first told investigators that he heard the child fall twice from a toilet, but the boy’s injuries were not consistent with that story. The medical examiner ruled Robert died from blunt-force head injury.

Durham was a paroled sex offender who had been sentenced to four years prison in 2003.

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