The Daily Herald reports that Sean Christopher Wright pleaded not guilty when he was charged in December with one count of first-degree sexual misconduct.
In court papers Thursday, prosecutors said additional evidence has come to light and they no longer believe they can prove the case at trial.
Prosecuting Attorney Mark Roe declined to offer more specifics.
Defense lawyer Mark Mestel says prosecutors did not tell him their reasons for dropping the charge.
Sheriff's spokeswoman Shari Ireton says Wright was placed on administrative leave with pay in June 2013, and remains in that status pending the results of an internal investigation.