MIAMI — In both word and deed, Sheriff Grady Judd of Polk County has long approached crime fighting as a moral quest, a clear-cut fight between good and evil, with no-nonsense punishments for offenders.

A church-going Baptist who has been handily re-elected twice, Sheriff Judd has battled prostitution and pornography and spearheaded an effort to shutter every sex-related business in the county in Central Florida. He gave away the jail yard’s basketball hoops because “if you want to play basketball, stay out of jail,” as he once said, and put a stop to buying underwear for inmates. At the moment, his office is in court over its treatment of juveniles in detention, one of many lawsuits he has faced over suspected civil rights violations.

But on Wednesday, his latest effort to root out wrongdoers — this time, child cyberbullies — came unraveled. Polk County prosecutors decided to drop felony charges against two girls in a September cyberbullying case that the police said contributed to the death of 12-year-old Rebecca Ann Sedwick, who jumped from a cement plant tower two months ago.

The decision was made by the Polk County state attorney’s office after weeks of investigation and an analysis of thousands of Facebook messages failed to turn up enough evidence to charge the girls, one 14 and the other 12.