Spokeswoman Lori Scamahorn said Rupe died Tuesday at the hospital inside the prison grounds. He had been there since Jan. 3
Rupe shot two Olympia bank tellers to death at point-blank range during a 1981 robbery.
Juries twice sentenced him to death, but higher courts overturned the sentences. In 1994, a federal judge upheld his conviction but agreed with Rupe's contention that at 400 pounds, he was too heavy to hang because of the risk of decapitation. Rupe argued that would be cruel and unusual punishment.
At the time, Washington's only manner of execution was hanging. The main method now is lethal injection.
Prosecutors tried for the death penalty a third time in 2000, but a jury deadlocked 11-1 — just shy of the unanimous vote required for capital punishment.
Rupe suffered from a terminal liver disease, and there was doubt at that time whether he would have lived long enough to be executed even if the jury had been unanimous.