SAN FRANCISCO -- The California Supreme Court Monday rejected a key appeal claim of a man given the death penalty for murdering two followers of the Grateful Dead band at a homeless encampment in Berkeley in 1985.

The high court, in a decision issued in San Francisco, by 6-1 vote turned down a claim by Ralph International Thomas that alleged incompetence by his defense lawyer affected the outcome of his trial.

Thomas was convicted in Alameda County Superior Court of murdering Mary Gioia, 22, and Greg Kniffin, 18, by beating them and shooting them at close range during the night of August 15-16, 1985.

The victims were so-called "Deadheads," or followers of the Grateful Dead. They were killed in Rainbow Village, a former homeless encampment set up by the city of Berkeley near the Berkeley Marina.

Thomas claimed in a habeas corpus petition that his defense attorney was incompetent in failing to investigate several witnesses who might have shown that someone else shot the victims.

The state Supreme Court concluded that the defense attorney was deficient in failing to find one of the witnesses, but said it wouldn't have made a difference in the trial.

Justice Kathryn Werdegar wrote that even if the additional witness had testified, "listening to the prosecution case would have established in a reasonable juror's mind the near certainty that Thomas did kill them."

Thomas still has several additional avenues of appeal available, including other claims in his state habeas corpus petition and then a federal habeas corpus petition.