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The 20-year-old Washington state man accused of killing five people in a shooting spree at a mall last September was found dead in his jail cell on Sunday night, authorities said.

The body of Arcan Cetin was found hanging just before 9 p.m. (12 a.m. ET), Skagit County Deputy Prosecuting Attorney Rosemary Kaholokula told NBC News Monday.

Cetin’s death hasn’t yet officially been ruled a suicide, Kaholokula said, but authorities believe it was.

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He had been charged with five counts of aggravated first-degree murder after he allegedly walked into a Macy’s in Burlington, north of Seattle, and opened fire with a rifle on the night of Sept. 23.

One man and four women were killed, including one who was trying to hide behind a cosmetics counter, NBC affiliate KING reported.

Cetin was a Turkish immigrant and naturalized American citizen who lived roughly 30 miles west of the mall, in the small seaside community of Oak Harbor, according to KING.

Kaholokula said Cetin had admitted to the killings, but his motive remained unclear.

The Washington State Patrol released this photo of Arcan Cetin, 20, of Oak Harbor, Washington after he was arrested Saturday, Sept. 24, 2016, in a shooting at a Burlington mall Friday that left five people dead.

“He didn’t have any really ties to our community,” she said. “We can’t ascertain why he would do this. He didn’t know any of the people that he shot.”

His stepfather, David Marshall, told reporters last year that Cetin had unspecified mental health issues that his family had been trying to address.

Before the shooting, KING reported, a local hospital tried to have Cetin involuntarily committed for treatment. In court documents obtained by the station, doctors said they believed Cetin might harm himself, though they didn't think he would hurt others.