A former Harvard University assistant track coach who also coached at a suburban Boston high school has pleaded guilty to possession of child pornography.

The U.S. attorney's office in Boston says 71-year-old Walter Johnson, of Framingham, pleaded guilty Tuesday. He faces up to 20 years in prison at sentencing scheduled for Sept. 10.

Authorities say an investigation began in January 2017 into child pornography being traded by a Craigslist user. The investigation led to Johnson, a track coach at Wellesley High School at the time. Authorities say during a search of Johnson's home, images and videos of child pornography were found on his computer and on a thumb drive hidden in his bedroom.

Johnson was an assistant coach of the Harvard men's and women's track teams from 1982 until 2006.