Thirty-year-old Justin Crouse is expected to be sentenced next month to between 20 and 25 years in prison, after pleading guilty in State Supreme Court on Friday to having killed his girlfriend’s son on the Cattaraugus Indian Reservation in March, court officials reported.

Crouse pleaded guilty to first-degree manslaughter before State Supreme Court Justice Penny M. Wolfgang, accepting a deal from the Erie County District Attorney’s Office.

Originally charged with second-degree murder and first-degree manslaughter in the death of 3-year-old Gage Seneca, Crouse pleaded to the manslaughter charge on the condition that he be sentenced to between 20 and 25 years, court officials said.

During his court appearance, authorities said, Wolfgang asked Crouse whether he intended to cause physical injury to the boy and whether he disregarded the risk to the child’s life. Crouse answered “yes” to both questions.

He will be sentenced Sept. 6.

Gage’s mother discovered the boy was not breathing when she checked his bedroom the morning of March 5. An autopsy determined the boy died as the result of blunt-force trauma to the head.

James F. Bargnesi prosecuted the case for the District Attorney’s Office, while Crouse was represented by Emily Trott and Robert M. Goldstein.