AUGUSTA – An Augusta man accused of choking, beating and stabbing his 74-year-old mother-in-law after an argument over a snowblower was convicted of murder Friday in Kennebec County Superior Court.

Jurors deliberated less than three hours before finding David Grant guilty in the November 2004 killing of Janet Hagerthy in Farmingdale.

The defense didn’t dispute that Grant inflicted the fatal injuries but told jurors the defendant was suffering from psychosis. Jurors were given the option of finding him guilty of a lesser charge of manslaughter.

Prosecutors said the fatal attack was sparked by a disagreement between Grant and Hagerthy over the proper way of starting the snowblower.

Afterward, Grant bound her body and drove around with it in his pickup truck before dumping the body in a field near her Farmingdale home.

Grant was arrested 10 hours after the episode when he crashed his pickup truck in the median of Interstate 95 in Newport. After the crash, the self-employed carpenter stabbed himself in the neck, requiring hospital treatment.

Defense lawyer Christopher MacLean contended Grant suffered from psychosis brought on by heavy cocaine use. He said Grant was intoxicated with cocaine and was out of touch with reality when he killed Hagerthy.

Police officers and a crisis worker testified that in the weeks before the killing, Grant reported children running around on the roof of his garage and peeking in his window.

But prosecutor Andrew Benson said Grant knew what he was doing when he bludgeoned and stabbed Hagerthy. The fatal injury was a stab wound.

“Intoxication is not a defense,” Benson said. “The very nature, the fact he changed methods shows very clearly he was still able to engage in intentional conduct. It was his conscious object to cause her death.”

Carlyle Voss, a psychiatrist who examined Grant at the request of the defense, testified Grant was not in control of his actions.

“He had no intent to harm her,” Voss said. “I think there was an altercation and he went ballistic and she died tragically.”

Murder carries a sentence of 25 years to life in state prison.

Grant also served time in prison after a 1988 conviction for attacking his estranged wife. He was accused of trying to suffocate the woman with a plastic bag and trying to drown her by holding her head underwater.