The Seattle Times reported that Jarod Lane faces up to 33 years in prison when he is sentenced in the May 2012 death of Jessica Scholl, 17.
Lane entered his pleas Thursday.
Friends said Lane and Scholl had dated for a couple of years and that Lane had seemed distraught over their breakup.
Firefighters found Scholl's body after they put out a fire on the second floor of her house. An autopsy showed she died from multiple stab wounds, blunt force injuries and possible strangulation.
Lane was arrested three days later in Oklahoma.
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