Sheriff Judd said Rebecca had been using these messaging applications to send and receive texts and photographs. His office showed Ms. Norman the messages and photos, including one of Rebecca with razor blades on her arms and cuts on her body. The texts were full of hate, her mother said: “Why are you still alive?” “You’re ugly.”

One said, “Can u die please?” To which Rebecca responded, with a flash of resilience, “Nope but I can live.” Her family said the bullying began with a dispute over a boy Rebecca dated for a while. But Rebecca had stopped seeing him, they said.

Rebecca was not nearly as resilient as she was letting on. Not long before her death, she had clicked on questions online that explored suicide. “How many Advil do you have to take to die?”

In hindsight, Ms. Norman wonders whether Rebecca kept her distress from her family because she feared her mother might take away her cellphone again.

“Maybe she thought she could handle it on her own,” Ms. Norman said.

It is impossible to be certain what role the online abuse may have played in her death. But cyberbullying experts said cellphone messaging applications are proliferating so quickly that it is increasingly difficult for parents to keep pace with their children’s complex digital lives.

“It’s a whole new culture, and the thing is that as adults, we don’t know anything about it because it’s changing every single day,” said Denise Marzullo, the chief executive of Mental Health America of Northeast Florida in Jacksonville, who works with the schools there on bullying issues.