Sheriff Grady Judd said 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.
Sedwick was "absolutely terrorised on social media," Judd said.
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?"
"If we can get any evidence of a criminal offense, the person or persons involved will be punished," he said.
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Before her death, Sedwick had searched questions online related to suicide, including "How many over-the-counter drugs do you take to die?" and "How many Advil do you have to take to die?"
Detectives also found photos of Sedwick with razor blades lying on her arms and with her head resting on a railroad track, Judd said.
Sedwick also changed her name on a free messaging application to "That Dead Girl."
Judd said detectives are trying to investigate the social media applications that Sedwick used, including Kik and Ask.Fm, but many of the websites are based in other countries.
If detectives can find evidence, according to Florida law, the girls could be charged with felony cyber stalking because Sedwick was under 16 years old.