Polk County Sheriff Grady Judd repeated the older girl's Facebook comment almost word for word at a news conference Tuesday.
"'Yes, I bullied Rebecca and she killed herself but I don't give a ...' and you can add the last word yourself," Judd said.
But in an interview with ABC News that aired Wednesday, a couple identified as the 14-year-old's parents said their daughter would never write something like that and that the girl's Facebook page had been hacked.
Of the claim that the 14-year-old's page was hacked, Judd said Tuesday: "Of course, we don't believe that."
Authorities in central Florida said Rebecca was tormented online and at school by as many as 15 girls before she climbed a tower at an abandoned concrete plant and hurled herself to her death Sept. 9. But the two girls arrested were primarily the ones who bullied Rebecca, the sheriff said. They have been charged with stalking and released to their parents.
Rebecca is one of at least a dozen or so suicides in the past three years that were attributed at least in part to cyberbullying.
The sheriff said they were still investigating the girls, and trying to decide whether the parents should be charged.
"I'm aggravated that the parents aren't doing what parents should do," the sheriff said. "Responsible parents take disciplinary action."
About a year ago, the older girl threatened to fight Rebecca while they were sixth-graders at Crystal Lake Middle School and told her "to drink bleach and die," the sheriff said. She also convinced the younger girl to bully Rebecca, even though they had been best friends.
The girls repeatedly intimidated Rebecca and called her names, the sheriff said, and at one point, the younger girl even beat up Rebecca at school.
Both girls were charged as juveniles with third-degree felony aggravated stalking. If convicted, it's not clear how much time, if any, the girls would spend in juvenile detention because they did not have any previous criminal history, the sheriff said.
The sheriff's office identified the two girls, but The Associated Press generally does not name juveniles charged with crimes.
The bullying began after the 14-year-old girl started dating a boy Rebecca had been seeing, the sheriff said.
A man who answered the phone at the 14-year-old's Lakeland home said he was her father and told The Associated Press "none of it's true."
"My daughter's a good girl and I'm 100 percent sure that whatever they're saying about my daughter is not true," he said.
At their mobile home, a barking pit bull stood guard and no one came outside despite shouts from reporters for an interview.
Neighbor George Colom said he had never interacted with the girl but noticed her playing roughly with other children on the street.
"Kids getting beat up, kids crying," Colom said. "The kids hang loose unsupervised all the time."
A telephone message left at the 12-year-old girl's home was not immediately returned and no one answered the door.
Orlando attorney David Hill said detectives may be able to pursue contributing to the delinquency of a minor charge for the parents, if they knew their daughters were bullying Rebecca yet did nothing about it.
But it "will be easy to defend since the parents are going to say, 'We didn't know anything about it,'" said Hill, who is not involved in the case.
Perry Aftab, a New Jersey-based lawyer, told AP last month that it is difficult to bring charges against someone accused of driving a person to suicide, in part because of free-speech laws.
The case has illustrated, once more, the ways in which youngsters are using the Internet to torment others.
In a review of news articles last month, AP found about a dozen suicides in the U.S. since October 2010 that were attributed at least in part to cyberbullying. Aftab said she thought the number was at least twice that.
Before her death, Rebecca changed one of her online screen names to "That Dead Girl" and she messaged a boy in North Carolina: "I'm jumping." Detectives found some of her diaries at her home, and she talked of how depressed she was about the situation.
Last December, Rebecca was hospitalized for three days after cutting her wrists because of what she said was bullying, according to the sheriff. Later, after Rebecca complained that she had been pushed in the hallway and that another girl wanted to fight her, Rebecca's mother began home-schooling her in Lakeland, a city of about 100,000 midway between Tampa and Orlando, Judd said.
This fall, Rebecca started at a new school, but the bullying continued online, authorities said.
"Rebecca's mother went above and beyond to create interventions. The one issue that Rebecca's mom said to us was, 'I just didn't want to have her not like me, so I wanted to give her access to her cellphone so she could talk to her friends,'" Judd said. "Rebecca's family is absolutely devastated by this. Quite frankly, we're all devastated by this."
Kay reported from Miami.