Arcan Cetin is suspected of killing four women and a man Friday night at the Cascade Mall in Burlington, Washington.
Washington state authorities said it's too early to rule out terrorism or anything else because their investigation is still in the preliminary stages.
Before the suspect's arrest, an official with the FBI had told reporters there was "no evidence at this time" of a link to terrorism.
When police confronted the suspect walking down the street near his residence in Oak Harbor, he did not try to run, he just froze and complied, said Lt. Mike Hawley of the Island County Sheriff's Office.
The suspect, who was not armed at the time of his arrest, said nothing, "kind of zombie-like," Hawley said.
Cetin immigrated from Turkey and is a legal permanent resident of the United States, authorities said. It's unclear when he moved to the US.
The shooting stunned the small town as authorities warned residents to stay indoors and be on alert Friday night. The gunman vanished into the night along Interstate 5 after the attack, according to state patrol Sgt. Mark Francis
For much of Saturday, authorities searched the mall for evidence. A blurry surveillance photo of a rifle-toting man inside the mall offered a big lead in the hunt for the shooter.
"The city of Burlington has probably changed forever," Mayor Steve Sexton said. "This is a senseless act. It was the world knocking on our doorstep, and it came to our little community here."
Authorities say they believe only one person fired the shots that killed five people.
The grainy security camera footage showed the suspect initially appearing to enter the mall unarmed and -- about 10 minutes later -- walking into Macy's carrying a rifle.
The suspect "fired multiple times, struck four females ranging in age from a teenager to seniors," said Mount Vernon police Lt. Chris Cammock.
It's unclear whether the suspect knew his victims.
"I don't know what his motivation was," Cammock said. "I don't know what his motivation was to continue, I don't know what his motivation was to stop, but I certainly intend to find out."
He said police recovered a rifle at the mall but would not disclose the type or caliber.
The FBI office in Seattle is helping with a review of intelligence. There's no information to suggest additional attacks were planned in the state, it tweeted.
Moviegoers asked to leave
Witnesses described chaotic scenes and confusion when the shooting started about 7 p.m. Friday local time.
Brandi Montreuil told CNN she was watching a movie at the mall when attendants suddenly told them to leave.
"I didn't know anything," she said. "The theater attendant came in and apologized for stopping the movie and said they were asked to have everyone leave immediately."
At first, she thought it was a drill.
"But you immediately think about what happened in Aurora, Colorado, so you start moving faster," she said, referring to the 2012 shooting at a movie theater
that left 12 dead.
When they got outside, they saw police vehicles but no officers in sight, Montreuil said.
"We didn't know if it was a fire so we were looking for smoke of some kind. Then a few officers started canvassing through the crowds, asking if anybody saw anything."
An officer "with a large gun started yelling for people to leave and fast," she said.
Armando Patino said he was working at a T-Mobile store near Macy's when he heard the commotion.
"Out of nowhere I just hear somebody yell and then after that, I turn around and just look at the Macy's and I just hear shots," Patino told CNN affiliate KOMO-TV
"I hear one shot and then stand kinda still and like two, three other people start running out saying, 'gun.' "
Some people dashed out of Macy's, unsure of where to go.
"I just moved them into the (T-Mobile) store," Patino said.
"We went in the back where we have a door ... and we just stayed there until they told us to evacuate."
Officers using K-9 units searched the 434,000-square feet mall throughout the night, including in secure rooms, authorities said.
Victims' identities not released
Washington Gov. Jay Inslee sent his condolences to the families of the victims and prayers to those injured.
The bodies were removed from the mall Saturday and are in the possession of the Skagit County Coroner's Office.
The coroner's office is identifying the victims and notifying their families before the names are released publicly.