A Fairmont man was arrested on Tuesday after allegedly engaging in sexual relations with a 13-year-old girl in Nicollet County. 33 year-old Matthew James Martinson was taken into custody after North Mankato police received a report of a suspicious vehicle on Rolling Green Lane. When officers arrived, they observed suspicious activity inside the vehicle, and confronted Martinson and a female occupant, who appeared to be under age. Authorities later determined that the girl was 13-years old. Martinson is being held in the Nicollet County Jail, and faces charges of 1st, 2nd, 3rd and 4th-degree criminal sexual conduct.