ROCKLAND, Maine — The state’s highest court has overturned the conviction of a Maine State Prison inmate accused of possessing a shank, or makeshift knife, after a fight in which another prisoner was injured.

The Maine Supreme Judicial Court ruled May 29 that testimony by a prison investigator, who stated that Timothy Mooney would have faced more serious charges if another prisoner had cooperated, should not have been presented to the jurors.

Following a jury trial in May 2011 in Knox County Superior Court, Mooney, 51, was convicted of possession of prison contraband. The contraband in this case was a shank, a 3-inch piece of metal used as a homemade knife. Mooney was sentenced to an additional three years in prison for the contraband possession conviction.

Mooney is serving a 35-year sentence for the 1998 murder of his former girlfriend Elizabeth Nelson-Blais. In 1998, Mooney was homeless and living in Portland. Police said he smashed the woman’s skull with a concrete block after the two had consumed vodka and orange soda in a metal shack.

In November 2010, prison guards broke up a fight between Mooney and another prisoner. A guard reported that when she pulled Mooney away from the other inmate she saw something fly past her. The shank was later found near the cell.

Mooney was not charged with stabbing the other inmate.

During the May 2011 trial, John Scheid, a criminal investigator for the Maine Department of Corrections assigned to the Maine State Prison, testified under questioning from the district attorney’s office that he would have sought additional charges against Mooney if the injured inmate had cooperated with investigators.

Mooney, who served as his own attorney at that trial, objected to the statement being allowed into evidence. Justice Joyce Wheeler overruled the objection.

“Although the trial court immediately and appropriately instructed the jury to consider only the charge pending before them at trial, we are unpersuaded that the substantial unfair prejudice created by the inadmissible testimony was avoided by the instruction,” the Maine Supreme Judicial Court ruling states.

The high court heard arguments on Mooney’s appeal last month. He was represented by attorney Thomas Shehan Jr. of Belfast.

The high court’s ruling vacated the judgement.

Assistant District Attorney Christopher Fernald, who argued before the court on behalf of the state, was not immediately available Tuesday afternoon to comment on whether the state would try Mooney again on the contraband charge.