Arcan Cetin, 20, who was awaiting trial in the case, apparently committed suicide and was discovered in his cell late Sunday.
The Turkish-born Cetin was charged with five counts of first-degree murder after the September 23, 2016 killings that left a teenage girl, three women and a man dead.
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The FBI at the time said there was no evidence the shooting was terror-related and Cetin's father told media that his son had mental health issues.
Cetin admitted in interviews with investigators to the killing spree at the Cascade shopping mall but gave no motive.
Local media said Cetin was awaiting a second mental-competency evaluation while being held at the Snohomish County jail.
He faced the death penalty if found guilty.
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