In a hearing at King County Courthouse, a judge ruled that the state cannot seek the death penalty against co-defendants Joseph McEnroe and Michelle Anderson.
The pair are on trial separately for the shooting deaths of six members of Anderson's family, which happened on Christmas Eve 2007 in Carnation.
Jury selection for McEnroe's trial is set to begin next week. McEnroe and Anderson were each charged with six counts of aggravated murder.
Statement by King County Prosecutor Dan Satterberg on the ruling:
We are deeply sympathetic to the families that have waited more than five years for this case to go to trial.
We will appeal today's decision to remove the death penalty in this case. We believe it is wrong. We will appeal on behalf of the six lives lost in this crime and because of the potential impact on all aggravated murder cases throughout the state - past, present and future.