WINTER HAVEN, Fla. — Two Florida girls who were primarily responsible for bullying a 12-year-old girl who killed herself were arrested after one of them acknowledged the harassment online, a sheriff said Tuesday.Police in central Florida have been investigating the death of 12-year-old Rebecca Sedwick, who climbed a tower at an abandoned concrete plant Sept. 9 and hurled herself to her death. Authorities said as many as 15 girls may have bullied Rebecca and the investigation was continuing. Polk County Sheriff Grady Judd said the arrests of the girls, ages 14 and 12, were hastened when the older girl posted Saturday on Facebook, saying she bullied Rebecca but she didn’t care. “We decided that we can’t leave her out there. Who else is she going to torment, who else is she going to harass?” Judd said.The 14-year-old girl was accused of threatening to beat up Rebecca while they were sixth-graders at Crystal Lake Middle School, telling her “to drink bleach and die” and saying she should kill herself, the sheriff said. The older girl convinced the younger girl to bully Rebecca, and they both repeatedly intimidated her, called her names and once the younger girl even beat Rebecca up, police said.