Investigators were working to determine what led a gunman to open fire and kill five people in a department store at a Washington state mall, police said on Sunday after arresting a 20-year-old suspect in the deadly rampage.

Police took Turkish-born Arcan Cetin into custody on Sept 24 in Oak Harbor, about 30 miles southwest of Burlington where the shooting occurred on Sept 23.

Cetin's demeanor when apprehended was "zombie like," police said at a news conference. He was unarmed and did not run from officers, they said.

A motive for the rampage remains unclear, and Cetin, who is due to appear in court on Monday, has not been charged.

The FBI said while they had no indication the attack was a "terrorism act", it could not rule out that possibility.

Cetin, who police said is a legal, permanent resident of the United States, is accused of opening fire in the cosmetics section of a Macy's department store at Cascade Mall, killing four women and a man.

Surveillance video showed the suspect entering the mall without a rifle, but he was later spotted on video in the store brandishing a weapon, police said. The rifle was recovered at the mall.

The attack followed a series of violent outbursts at shopping centers across the United States, including the stabbing of nine people at a Minnesota center last weekend.

Investigators planned to search Cetin's Oak Harbor residence, vehicle and interview witnesses on Sunday to collect evidence and "build as good as a case as you can", said Sergeant Mark Francis, a spokesman for the Washington State Police.

Police have reports that Cetin's ex-girlfriend worked at the Macy's, Francis said. The possible connection was under investigation.

Cetin has a criminal record that includes three domestic violence charges in which his stepfather was the victim, the Seattle Times reported, citing court records.

Reuters was unable to confirm the reports.

The Daily Mail of London identified the victims as Beatrice Dotson, 95, and her daughter Belinda Galde; Chuck Eagan, a father of two; Shayla Martin, a makeup artist as Macy's; and Sarai Lara, 16, a cancer survivor.

(China Daily USA 09/26/2016 page1)