Arcan Cetin, the man charged with killing five people in a shooting at Cascade Mall in Burlington last September, was found dead Sunday night at the Snohomish County Jail.

The Skagit County Prosecutor's Office said Cetin (pronounced AR'-jahn CHEHT-in) was discovered hanging in his cell just before 7 p.m. Officers attempted CPR but were unsuccessful.

"I feel we were seriously robbed of justice by him taking his own life," said Amy Martin, whose aunt Shayla Martin was one of the victims. "I think that's the coward's way out.

Cetin, 20, was charged with five counts of aggravated first-degree murder for the Sept. 23, 2016 killings. The charges alleged Cetin acted with "premeditated intent" when he used a rifle to kill a teenage girl, three women, and a man at the Macy's women's store inside the mall 65 miles north of Seattle.

"On some level there is a level of disappointment. We would have liked for this case to have gone through the entire proceeding and to have justice after we were able to present our case," said prosecutor Rosemary Kaholokula.

Cetin's stepfather, David Marshall, previously told the media that Cetin "has had mental health issues that we have been working on him with."

The 20-year-old had a history of violent behavior toward his family and ex-girlfriend. His neighbor described Cetin as "creepy."

Cetin had visited his mother and Marshall before the Sept. 23 shooting, and stolen his stepfather's .22-caliber Ruger rifle, which had been hidden under a bed, according to records acquired by The Associated Press.

On the night of the shooting, he first went to the mall's movie theater and used his cell phone to prop the door open when he went outside. But another movie-goer closed the door behind him and gave the phone to theater officials, thwarting whatever plans Cetin may have had.

Soon after, Cetin retrieved his phone from officials and went into the mall.

Cetin entered through the Chuck E. Cheese restaurant just before 7 p.m. and headed for the Macy's women's store, Mount Vernon police officer Dave Shackleton said in an arrest warrant affidavit.

Surveillance video shows Cetin shooting a teenage girl near some clothing racks and then shooting a man. He then headed for the cosmetics section where he shot two women who were hugging and trembling in front of the counter and one who was trying to hide behind it.

Cetin left the rifle with a 25-round magazine on a cosmetics counter before fleeing, police said in court documents. The victims ranged from a 16-year-old girl to a woman in her 90s and included a probation officer and a Boeing maintenance worker.

Authorities circulated a photo of the shooter taken by a surveillance camera and soon identified Cetin as that man.

He was arrested about 30 hours later as he walked near his apartment in Oak Harbor. He had been hiding in Seattle and sleeping in his car while on the run, records showed.

During interviews with police, Cetin admitted that he shot the five people at the mall, but did not provide a reason.

Now, no one will ever know.

"It just isn't fair and now not knowing why kind of makes it worse," said Martin.

Cetin was in the midst of mental health evaluations to determine whether he was fit to stand trial. He had been moved from the Skagit County jail to Snohomish County because he was making comments detrimental to his case to other inmates at the Mount Vernon facility. That created a potential conflict of interest for Cetin's public defenders who might have had to put people they are actively defending on the witness stand.

Court documents show Cetin attempted suicide back in 2015.

Many unanswered questions remain about his death in Snohomish County such as, was Cetin under a suicide watch, when was the last time he was checked on, and how was he ultimately able to take his own life?

This is at least the 14th inmate death at the Snohomish County jail since 2010.

The family of shooting victim Wilton "Chuck" Eagan released the following statement on Monday: “We’re understandably in shock over this development. Our heart goes out to Mr. Cetin’s family. We pray that the man repented to God before his death. While this event puts to rest our fear of his release, we harbor no ill will towards Mr. Cetin or his family and pray for their comfort as we know all too well the pain of grief. The family wishes to extend our continued thanks to those who responded to this tragedy, especially law enforcement and the Skagit County Prosecutor’s office for their steadfast professionalism and compassion. We are also grateful to all of those in the community whose love has helped see us through the past six months as we grieve and remember Chuck. The family requests privacy as we continue to process this turn of events.”

The Associated Press contributed.