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(CNN) -- Rebecca Sedwick should be 13 now. She should be enjoying the birthday cupcakes and ice cream her mom planned for her.

But Rebecca will never know what it's like to be a teenager. She killed herself last month, jumping off a silo after she was tormented by bullying.

Her mother, Tricia Norman, posted a birthday message to Rebecca on Facebook on what would have been her 13th birthday Saturday.

"I remember so clearly the moment you were born. I knew I was gifted with a precious life and every moment since, I did my very best to protect you and now I feel like I failed that very important job that God gave me because He took you from me way too soon," Norman wrote.

"I am doing my very best to make sure that other parents are made aware of how serious of a problem bullying is and I hope you are proud of me and satisfied with my progress. I am also working hard to make sure those that wronged you pay the price for what they did to you!!"

Over a boy?

Rebecca and one of her alleged bullies were once friends, but they both had feelings for the same boy, police said. It soured their relationship.

In November 2012, the bullying began, Judd said, and it expanded beyond the initial tensions between the two girls.

Police arrested a second suspect accused of intensely bullying Rebecca, and they also confiscated the laptops and cell phones of 15 girls at Crystal Lake Middle School, which Rebecca had attended.

They found a barrage of horrible messages:

"nobody cares about u"

"i hate u"

"you seriously deserve to die"

Rebecca apparently gave up on herself, Polk County Sheriff Grady Judd said. "Rebecca wasn't attacking back. She appeared to be beat down."

But Rebecca's suffering was no secret at her school. Teachers saw her in tears. In December, she was hospitalized after slitting her wrists.

Her school started a campaign against bullying, giving talks before the student body to discourage it.

The night before her deadly plunge, she sent a message to a boy she met on Facebook: "I'm jumping. I can't take it anymore."

A suspect's defense

Two girls have been charged with aggravated stalking in Rebecca's case: 14-year-old Guadalupe Shaw and 12-year-old Kaitlyn Roman.

CNN does not typically identify minors who are charged as juveniles, but the network is doing so in this case because their identities have been publicized by law enforcement and have received extensive publicity in their local media.

Weeks after Rebecca leaped to her death, a potentially incriminating message appeared on Guadalupe's Facebook page:

"Yes IK I bullied REBECCA nd she killed her self but IDGAF," the message read. Judd said the online vernacular meant: "I don't give a (expletive)."

The sheriff called in Guadalupe and her family about the post. Police arrested the daughter.

But Guadalupe's father, Jose, told CNN's "New Day" last week that his daughter couldn't have been the one to post that message.

The father had just finished watching the news on TV when he found her asleep and sent her to bed. It was about that time that the message landed on Facebook, he said.

"I grabbed the computer and took it to my room, and the only other thing she could have used to send this message was my cell phone, and my cell phone is always with me," he said.

He said he suspects someone hacked her account.

But the sheriff said he doubts that. Judd said investigators don't believe the teen's Facebook account was compromised, Judd said.

Sheriff: Taunting post leads to arrests in bullying death

Mother of teen suspect arrested

Not long after Guadalupe's arrest, her mother was arrested on suspicion of child abuse and child neglect, authorities said Friday.

Judd said 30-year-old Vivian Vosburg was arrested Friday afternoon.

The sheriff emphasized that Vosburg's charges are not related to those of her daughter and the other young suspect in Rebecca's case.

Vosburg's arrest came after authorities received tips from residents that she might have been involved in beating children, Judd said. A minute-long video clip Judd played at a Friday press conference showed a woman, whom he identified as Vosburg, punching two boys with her fist while shouting profanities.

Judd said the mother first argued the beating was an accident but then said it "got out of hand" partly because she was having a bad day.

The video had been posted to the Internet and has since been taken down. It wasn't immediately clear who took the video or when and where it was taken.

Boy asks Santa to make kids stop bullying his sister

CNN's Kim Segal, John Couwels and Ben Brumfield contributed to this report.