A 21-year-old man who sexually assaulted an intellectually impaired teen while both resided in a Ledyard foster home has been sentenced in New London Superior Court to 10 years of strict probation and registration as a sexual offender.

Matthew R. Olsen had pleaded guilty to risk of injury to a minor.

Judge Susan B. Handy sentenced Olsen last week after he completed a longterm residential treatment program at the Stevens Children’s Home in Swansea, Mass., according to prosecutor Theresa Anne Ferryman.

Working with attorney Peter E. Scillieri from the public defender’s office and officials from the state Department of Children and Families and state Department of Mental Health and Addiction Services, the court officials worked out a resolution that they hope will enable Olsen to return to Massachusetts to receive further treatment.

According to an arrest warrant affidavit, a foster mother from Ledyard called state police in October 2011 after walking in and seeing Olsen and the male victim, then 16, engaged in sexually inappropriate behavior.

Olsen confessed to police that he had sexual contact with the victim over the past three years. The victim, due to his impairment, was able to articulate only that he had been hurt by “Matt.”

Handy sentenced Olsen to five years in prison, fully suspended, and 10 years of sexual offender probation. He is required to register as a sex offender for the full term of his probation.

Arrested as a youthful offender, he was tried as an adult due to the seriousness of the charge.

He is living in Hartford, in the custody of DMHAS, while the Department of Adult Probation attempts to transfer his probation to Massachusetts.

k.florin@theday.com

Twitter: @KFLORIN