A state Department of Education investigation into a former Willmar High School teacher accused of a sexual relationship with a student determined that there wasn’t enough evidence to find maltreatment, according to a defense filing this week in the teacher’s criminal case.

Onetime Willmar coach and health teacher Chad J. Akerson, 34, was charged in December with sexual misconduct involving one of his students, who also was on his hockey team. Charges allege the sexual relationship started in early 2010, when the student was 15 and continued until well after she became an adult.

Akerson left the district in October 2012 and joined the high school teaching and coaching staff in Ashby, Minn., the next February. He was put on unpaid leave in Ashby after his Dec. 3 arrest.

Included in a defense motion filed this week was an August 2013 letter that Akerson received from an education department senior investigator, outlining the department’s investigation and stating that, based on all of the available information, “the preponderance of evidence does not support [that] the Teacher is responsible for maltreatment of the Student in the form of sexual abuse.”

The eight-page letter outlined what a school administrator, an officer and an attorney investigating the allegation recounted to the department, in addition to interviews with a family friend and a social worker.

According to the letter, the mother told the administrator and the officer:

She and Akerson became friends, dated and had a romantic relationship that was kept secret from the student due to the parents’ recent divorce and the student’s friendship with Akerson. The student was often alone with Akerson, whom the mom viewed as a “father figure.” After two years, the mother became suspicious that something sexual was going on, so she hid an audio recorder in her living room and bedroom, and captured a conversation in which her daughter and Akerson talked about sex.

The mother confronted her daughter, who confirmed that she had sexual contact with Akerson and had “used protection.” The parent later went to Akerson’s home to confront him, and Akerson initially denied having sex with her daughter but later admitted to it and said the student came on to him.

The mother ended her relationship with Akerson. She said Akerson said he would kill himself if he lost his job and had to go to jail.

The education department’s letter also outlined a recorded interview that the attorney had with the student, who said she viewed Akerson as a “father figure” and spent time alone with him at her mom’s urging. She said she lied to her mother about it being sexual after learning of their relationship in order to “one up” her mother and make her think that Akerson loved only her.

In the interview, which lasted an hour and 21 minutes and included the parent, the administrator, a witness and a social worker, the student maintained that she never had any physical contact with Akerson, the letter said.

In a phone interview, a family friend told a department investigator that the student had been worried about Akerson, saying the situation was her fault and that she loved him and would be with him after she graduated. The student refused to give information to an officer in October 2012, according to the letter. According to court papers, the student told police in November of a regular pattern of sexual intercourse with Akerson. She said their relationship continued until October 2014.

Akerson is expected to plead not guilty in court Monday, according to his attorney, Ryan Garry.