A St. Thomas police officer has pleaded guilty to sexual interference with a minor.

Const. Garry Christiansen, 57, admitted to becoming involved with a 15-year-old girl.

It began when the teen visited the police station for a background check.

In an agreed statement of facts, the court heard the then-53-year-old initiated a relationship with a 15-year-old girl, who had come in to the police station for a police background check.

After exchanging text messages, they were reportedly involved with one another for 17 months in secret.

They would meet at the officer's home, and while the encounters did not initially involve intercourse, that reportedly changed once the girl turned 16.

He had originally been facing five charges, including assault and sexual assault.

Christiansen resigned from the force on Monday.

He’ll be sentenced in September, when he could get up to 14 years in prison.

READ THE UPDATE: Christiansen was sentenced to 30 months in prison.

With files from CTV London.

Correction:

This story has been edited for clarity. A previous version suggested that their relationship began when the girl was 15. In fact, court documents show that they met when she was 15, but did not have intercourse until she turned 16.