TACOMA, Wash. - Some students who heard the gunshots or saw the victim crumple to the floor didn't believe what was happening was real. The screaming teachers and streaks of blood on the lockers quickly convinced them it was.

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The shooting, which happened just before the first period bell rang on the day classes resumed after the holiday break, sent students scrambling as 17-year-old Samnang Kok lay dying in a hallway. The shooter bolted out a set of double doors.

About two hours later Wednesday, police arrested fellow student Douglas Chanthabouly, 18, a few blocks away from Henry Foss High School. He was booked for investigation of first-degree murder.

The shooter and victim knew each other but detectives did not immediately know a motive, Tacoma Police Chief Donald Ramsdell said. He said the arrested boy was cooperative.

"Today we lost a nice young person in a Tacoma public school in an act of senseless violence," Ramsdell said. He vowed to work with the school district to "prevent this type of situation in the future."

Police said Chanthabouly had no criminal record. He was expected to make an initial court appearance Thursday. It was not immediately known if he had obtained a lawyer.

Sheriff's Detective Ed Troyer said the teen was on suicide watch, which is common in high-profile cases. Police said the shooting was not believed to be gang related or racially motivated.

When the shooting began, "I was 20 feet away," said the school's principal, Don Herbert. "The only thing I could have done was taken the shot instead. But it happened very fast."

An armed school resource officer was on the scene within seconds of the shooting and radioed police for help as teachers herded students into classrooms and the gymnasium, said Detective Chris Taylor. Three teams of about six officers, each armed with rifles, swept the school to ensure the gunman had left.

Two 15-year-old sophomores, Malcolm Clark and Josh Wilber, said they witnessed the shooting and were questioned by police.

"He got shot - bang - and he just fell," Clark said of the shooter and the victim. "He just froze and he fell backwards into the lockers."

Other students "were like, 'Nah, he's playing. He's going to get up,'" Wilber said. "And then the teachers started yelling 'Get in the classroom!'"

Kok died of three shots at point-blank range. He was the father of an 18-month old son, Makhai Kok, with his girlfriend, Tiari Johnson, 16. No one else was hurt in the 7:30 a.m. shooting.

About two hours later, a neighbor a few blocks from the school called police to say that someone matching the suspect's description was in the area. Officers arrested him without incident.

The school was locked down after the shooting. Classes were canceled for the rest of the day and students sent home about an hour later.

Gov. Chris Gregoire said the shooting was further evidence that the state needs to complete its school mapping program, an effort to provide emergency responders with up-to-date information about school layouts and evacuation routes.

All of the state's high schools have been mapped; 725 of Washington's public elementary and junior high schools have yet to be, she said.