The older of the two girls acknowledged to the police that she had bullied Rebecca. She said she had sent Rebecca a Facebook message saying that “nobody” liked her, the affidavit said. The girl also texted Rebecca that she wanted to “fight” her, the police said. But the bullying did not end there; Rebecca was told to “kill herself” and “drink bleach and die” among other things, the police added.

Image Rebecca Sedwick committed suicide last month after being repeatedly bullied in person and online.

The bullying contributed to Rebecca’s suicide, the sheriff said.

Brimming with outrage and incredulity, the sheriff said in a news conference on Tuesday that he was stunned by the older girl’s Saturday Facebook posting. But he reserved his harshest words for the girl’s parents for failing to monitor her behavior, after she had been questioned by the police, and for allowing her to keep her cellphone.

“I’m aggravated that the parents are not doing what parents should do: after she is questioned and involved in this, why does she even have a device?” Sheriff Judd said. “Parents, who instead of taking that device and smashing it into a thousand pieces in front of that child, say her account was hacked.”

The police said the dispute with Rebecca began over a boy. The older girl was upset that Rebecca had once dated her boyfriend, they said.

“She began to harass and ultimately torment Rebecca,” said the sheriff, describing the 14-year-old as a girl with a long history of bullying behavior.

The police said the older girl began to turn Rebecca’s friends against her, including her former best friend, the 12-year-old who was charged. She told anyone who tried to befriend Rebecca that they also would be bullied, the affidavit said.

The bullying leapt into the virtual world, Sheriff Judd said, and Rebecca began receiving sordid messages instructing her to “go kill yourself.” The police said Rebecca’s mother was reluctant to take her cellphone away because she did not want to alienate her daughter and wanted her to be able to communicate with her friends. Ms. Norman tried, she has said, to monitor Rebecca’s cellphone activity.