The Democrat said he came to the decision after months of review, meetings with family members of victims, prosecutors and law enforcement.
According to a draft statement obtained by The Associated Press, Inslee said that the use of the death penalty is inconsistent and unequal.
"Equal justice under the law is the state's primary responsibility," he said in the written statement. "And in death penalty cases, I'm not convinced equal justice is being served. The use of the death penalty in this state is unequally applied, sometimes dependent on the budget of the county where the crime occurred."
Inslee's moratorium means that if a death penalty case comes to his desk, he will issue a reprieve, which isn't a pardon and doesn't commute the sentences of those condemned to death.
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Nine men await execution at the Washington State Penitentiary in Walla Walla. The state Supreme Court just last month rejected a petition for release from death row inmate Jonathan Lee Gentry, sentenced for the murder of a 12-year-old girl in 1988. Gentry could be the first execution in the state since September 2010, when Cal Coburn Brown died by lethal injection for the 1991 murder of a Seattle-area woman.
The decision by the governor comes following a recent decision by the state Department of Corrections, which is in the process of changing its execution protocol to allow witnesses to executions to see the entire process, including the insertion of intravenous catheters during a lethal injection.