Jurors took about 30 minutes Wednesday to convict 19-year-old Jonathan Kurtz of murder and other charges in the November stabbings of Paul and Mara Skinner.
Prosecutors say Kurtz and his 19-year-old neighbor, James Preston, attacked the couple at their home in Yale, about 85 miles northeast of Detroit.
Preston and the couple's daughter, 18-year-old Tia Skinner, will face the same charges during trials later this summer.
Investigators say Tia Skinner was angry at her parents for forbidding her from seeing Kurtz and for confiscating her cellphone.
The Times Herald in Port Huron reports that Kurtz faces a mandatory life prison sentence.