A missing 15-year-old Louisiana girl may have run away with a much older man, authorities said.

Domeanna Spell was last seen getting off her school bus about 7 a.m. at Port Barre High School on Thursday.

She then got in a car driven by Cory "Shane" Disotell, a 47-year-old divorced father who is a family friend, her older sister Jerrie Cradeur told InsideEdition.com Tuesday. "He was in love with my sister. He wanted to marry her."

Domeanna hasn't been seen by her family since then, according to police. Her relatives are frantic with worry.

Cradeur said a student saw her walk across the street from the school and get into a car with a man believed to be Disotell. 

The girl's family learned months ago that she and Disotell had a secret relationship, Cradeur said. The man had hired Domeanna to babysit his granddaughter on weekends, she said. But the family learned he had no grandchildren.

Disotell gave the girl money to make it look like he was paying her to babysit, her sister said.

He told the girl's parents about six months ago he wanted to marry the teen, her sister said. "Everyone was pretty much hysterical," she said. The parents told him to never come back to their house and ordered their daughter to stay away from him, Cradeur said.

But they continued seeing each other, the sister said. About three weeks ago, the parents went to police after a neighbor's surveillance camera captured a man climbing over the Spell's back fence and going to Domeanna's bedroom window, Cradeur said.

The police were investigating when the two disappeared last week, Cradeur said.

The teen may have changed her hair color, Cradeur said, and replaced her eyeglasses with colored contact lenses.

The FBI has joined the search for Domeanna, aiding the U.S. Marshals Service, the St. Landry Parish Sheriff's Office, the Port Barre Police Department and the Louisiana State Police.

Anyone with information about the missing girl, who stands 5 feet, 2 inches tall and weighs 105 pounds, is asked to call 800-225-5324. Disotell is believed to be driving a silver 2003 Honda Civic with Louisiana plates. 

Cradeur said her little sister "was just in love with all the attention" she allegedly received from the older man and was brainwashed by him.

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