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A female substitute teacher who taught in a Wisconsin school district has been fired and is under investigation for an alleged sexual relationship with her daughter's 13-year-old boyfriend, FOXNews.com has learned.

The Pierce County Sheriff’s Department is investigating allegations that the 38-year-old Prescott School District substitute teacher had sex with the seventh grader, Lt. Dennis Sorenson said.

The investigation began after the student’s father filed a civil restraining order on behalf of his son against the woman following an incident that allegedly took place on May 17, said Sorenson, who is in charge of the investigation for Pierce County.

That evening, the father said he found the two having a “middle of the night rendezvous” together at the teacher’s home after the boy stole his mother’s car, WCCO-TV reported. The woman’s husband and 13-year-old daughter were home at the time, the station said.

The name of the teacher cannot be released because the investigation is still in its initial stages and no charges have been filed, Sorenson said.

The boy's father told the Prescott Journal that he believed the teacher and his son had sexual contact and sexual intercourse. In statements given during the process of obtaining the restraining order, he said the contact included kissing, touching each other while naked and exposing private parts.

The father also stated that his son confessed to being in the teacher’s home and to deleting e-mail messages the two had exchanged, the newspaper reported.

The father later said that he had already made copies of those messages and passed them on to investigators.

The 13-year-old boy’s seventh grade football coach told WCCO-TV that he was appalled by the alleged incident because of the boy’s relationship with the teacher’s daughter.

“He was dating the lady’s 13-year-old daughter before this happened,” Zach Simones told the station.

Roger Hulne, Prescott School District superintendent, said the substitute teacher had been employed by the district since the beginning of the school year and was officially terminated on May 18.

“I ended her employment the day after hearing about the allegations,” Hulne said.

The teacher was also once employed at St. Joseph’s Catholic School in Prescott, Wis., but that was “about two or three years ago,” Principal Mary Bray said. She declined to comment any further.

It is possible that the woman taught the seventh grader at some point during his academic career because she had substituted at all levels of the district, including the elementary school, high school and all three grades in the middle school, Hulne said.

An independent investigation conducted by the school determined that no other students were inappropriately involved with the woman.

“We have done and will continue do whatever is necessary to keep our schools safe,” Hulne said.