LAKELAND, Fla. -

Polk County Sheriff Grady Judd says a 12-year-old girl committed suicide after she was bullied online by more than a dozen girls, and he's investigating the cyber-bullying.

Judd said Thursday during a news conference that Rebecca Ann Sedwick jumped to her death on Monday at an old cement business. Investigators say the girl was despondent after others had posted hate messages about her online.

The Lakeland Ledger reports that detectives found multiple social media applications where Sedwick was constantly bullied with messages, including "Go kill yourself," and "Why are you still alive?"

Judd said Sedwick was terrorized.

He said parents of all 15 girls have cooperated with detectives and several cell phones and laptops have been confiscated.