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 say after arresting a 20-year-old suspect in the deadly rampage.
Police took Turkish-born Arcan Cetin into custody on Saturday evening in Oak Harbor, about 30 miles (48km) southwest of Burlington where the shooting occurred on Friday night.
Cetin's demeanour when apprehended was unarmed and "zombie like", police said at a news conference.
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 centres across the United States, including the stabbing of nine people at a Minnesota centre last weekend.
Investigators planned to search Cetin's Oak Harbor residence, vehicle and interview witnesses on Sunday to collect evidence, 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.
Police did not identify the victims but local media said they ranged in age from mid-teens to mid-90s, and included a mother and her daughter.
The Skagit County Coroner's Office said it planned to release information about shooting victims on Monday.