A Florida fitness guru who “wreaked havoc” on her competitors’ lives was sentenced to prison for using 369 dummy Instagram accounts to harass rivals and stage a kidnapping, officials said.

Tammy Steffen, 37, pleaded guilty on Friday to federal charges of cyberstalking and sending threatening communications online, according to the Department of Justice.

“The extent of her crime is astounding,” FBI special agent Kristin Rehler told the news station WFLA.

FBI agents said the mother of four used hundreds of bogus Instagram accounts — such as catloverexpress — and 18 email accounts to intimidate her ex-business partner at a Tampa gym and perceived rivals in the fitness world, according to the Washington Post.

“I plan to slice you up into little pieces,” said one message. “Your blood shall I taste.”

Officials said she told another victim that “all hell is gonna rain fire down on your world like never seen before” — then followed up with a photo of two knives with a caption that read, “I’m coming.”

Feeling jilted from the loss of an online fitness contest, Steffen had also plotted revenge against the ex-business partner, who she believed sabotaged her in the competition, officials said.

On July 9, 2018, Steffen called authorities to report that a headless baby doll was left on her porch with a chilling note.

Inside the doll, the note said, “New toy for the kids.”

Then, five days later, Steffen called 911 again claiming that someone tried to kidnap her daughter.

When authorities arrived, she showed them the wooded area where a man supposedly attempted to take the girl. She then suggested that her ex-business partner was a potential suspect.

While investigating the scene, authorities uncovered a laptop case hidden in the brush that contained a notepad with information about Steffen and her home, according to the Washington Post.

But officials began to question Steffen’s claims when her daughter asked “what would happen to [her mother] if she told the truth.”

She was arrested later that month on charges of filing a false police report, tampering with evidence and child neglect.

But the charges piled on when officials discovered that she made threats online against other fitness experts, including her ex-business partner, and tried to discredit their careers.

“In addition to sending threatening messages online, she spoofed calls, she also used voice-disguising software,” Rehler told WFLA.

Through her bogus social media accounts, the fitness trainer “traumatized victims and wreaked havoc on their lives,” he said.

“What we would say is, this investigation sends a clear message to those who think they can hide behind the internet and send threatening messages,” Rehler told WFLA.

Steffen was sentenced to four years and nine months in prison as part of a plea agreement with federal prosecutors.