Polk County Sheriff Grady Judd said that the girl jumped to her death on Monday at an old cement business in Lakeland. Investigators say the girl was despondent after others had posted hate messages about her online.
Sedwick was "absolutely terrorized 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?"
Judd said parents of all 15 girls have cooperated with detectives and several cellphones and laptops have been confiscated. He said charges could be filed - including cyber stalking.
"If we can get any evidence of a criminal offense, the person or persons involved will be punished," he said.
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.
Judd said a 12-year-old boy in North Carolina, who Sedwick met through social media, knew of her plan. Sedwick messaged him only hours before her death saying she was dead and "I'm jumping, I can't take it anymore."
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.
"If you bully somebody online and it's reported to us and we can build a credible case, we will charge you," Judd said.
Sedwick attended Crystal Lake Middle School, but relatives say she was pulled from the school, home schooled for a time, and had just recently attending a new school, all as a result of the bullying.