About 25 unreleased songs by the rapper were discovered online, and investigators traced sales of the music to a former employee, according to federal prosecutors.

A former sound engineer for award-winning rapper Eminem—also known as Marshall Mathers—was charged on Wednesday with stealing the rapper’s unreleased music and selling it for Bitcoin, according to federal prosecutors.

Joseph Strange, who worked for Slim Shady for more than a decade, faces a up to 15 years in prison if convicted.