Attorneys for convicted killer Ronald Lafferty will have an opportunity to show their client was mentally incompetent to stand trial for the slashing deaths of his sister-in-law and her 15-month-old daughter.

The Utah Supreme Court decided Tuesday to allow Lafferty's attorneys to present evidence contained in 550 pages of transcripts that previously had not been considered by the court.The attorneys contend the transcripts cast doubt on Lafferty's competency to stand trial, as well as on his understanding of what he was doing when he waived his right to use an insanity defense and on his original attorney's ability to present an effective defense.

State attorneys had argued the new transcripts contain nothing that would change the case.

Lafferty, sentenced to die for the 1984 murders, has a separate appeal pending in federal court. His attorneys hope the new evidence will eventually lead to a commutation of his death sentence.

Justices said the new evidence will not be heard before January.