Both Haag and Sibson were taken to the hospital where Haag died from her injuries.
Sibson and Haag were hospitalized after the shooting.
Sibson, whose blood-alcohol level was found to be three times the legal driving limit, "had an entry wound on the left side of his head, he had an exit wound on the top of his skull", but he was saved by the "extraordinary skill of heroic medical personnel", Fayette says. Sibson told investigators that he couldn't remember the mechanics of the shooting, Fayette said.
A judge on Sunday entered a not guilty plea on behalf of Sibson during a courtroom appearance, KTUU-TV reported.
Prosecutors believe Haag was trying to prevent Sibson from killing himself when she was shot. After considering first and second-degree murder charges against Sibson, a grand jury chose to indict him on the latter charge last week and he surrendered to authorities Friday, Alaska Dispatch News reports.
Sibson was jailed for lack of $250,000 bond. He turned himself in Friday after police issued an advisory saying officers were not able to find him for two days.
If convicted, Sibson faces up to 99 years in prison.
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