A Suffolk man on Thursday changed his plea to guilty in the case of his girlfriend’s death last year.

William Daniel Conroy Jr., 20, pleaded guilty to involuntary manslaughter, distribution of fentanyl and arson of an unoccupied building, which was a separate incident.

The plea agreement has not yet been officially accepted. Judge Carl Eason agreed to take it under advisement until the sentencing on Dec. 3. Conroy was initially charged with second-degree murder.

Conroy’s girlfriend, 17-year-old Briley Marie Compton, went into cardiac arrest in the backyard of his Conroy’s residence in the 2700 block of Country Club Drive on April 22, 2011, according to a city spokeswoman at the time Conroy was charged.

Police were told she had fallen from a boat in the backyard, but tests on the body revealed she died because of a combination of Coricidin, an over-the-counter cold medication, and a prescription narcotic, fentanyl, that was not prescribed for her. Conroy gave her the prescription, police said.

The arson charge stemmed from an incident about three weeks after Compton’s death. Conroy burned someone else’s duck blind, according to police.