A 19-year-old Alaska man has pleaded "guilty, but mentally ill" in the stabbing deaths of his grandparents, aunt and her companion.

Jason Abbott of Sitka pleaded guilty Thursday to two counts of first-degree murder in a deal with prosecutors.

Under the deal, prosecutors consolidated charges in the deaths of grandparents Alice and John Abbott as one count and the deaths of his aunt, Elizabeth Abbott, and her friend, Charles Tate, in a second count.

Abbott submitted the same plea to an assault charge in the stabbing of his aunt, Mary Ricketts, who survived the 2008 attack.

Abbott's attorney has said he was diagnosed by prison doctors with paranoia and severe schizophrenia.

He faces at least 40 years and one day in prison when sentenced Sept. 25.