PORTLAND – The state supreme court Thursday upheld an Augusta man’s conviction for murdering his mother-in-law, rejecting arguments that his hospital-room confession was obtained illegally.

The justices unanimously denied the appeal of David Grant, 57, who is serving a 70-year prison term for the slaying of Janet Hagerthy at her home in Farmingdale on Nov. 30, 2004. The retired nurse was choked, beaten and stabbed to death after an argument over how to start a snowblower.

In seeking a new trial, Grant maintained that detectives failed to honor his Miranda rights allowing him to remain silent or be represented by a lawyer.

The Supreme Judicial Court disagreed, noting that police immediately ceased their questioning when the suspect invoked his Miranda rights the day after the slaying. When detectives returned the next day, Grant was given fresh Miranda warnings but allowed the interview to take place.

The court said more than 19 hours had elapsed before the questioning resumed and Grant was not given new pain medication that could have diminished his lucidity.

“We conclude that taken as a whole, the conduct of the police did scrupulously honor Grant’s invocation of [the right to remain silent],” the opinion concluded.

The questioning took place while Grant was in a Bangor hospital for emergency surgery after being found in his pickup truck in a ditch in Palmyra, repeatedly stabbing himself in the neck with a knife.

The motion judge had refused to suppress Grant’s statements to police on grounds that he was in custody “only in a medical sense” when approached for questioning. The supreme court disagreed, saying that while Grant may not have known that officers were stationed outside his hospital room, he was aware that he was the focus of the investigation.