The trooper had stopped a pickup around 1 a.m. Thursday on Highway 16 near the small Kitsap County town of Gorst and radioed the location and license plate number, said Trooper Russ Winger.
When the trooper didn't respond to status checks, a Kitsap County sheriff's deputy went to the scene and found the wounded trooper. The trooper was taken St. Joseph Medical Center in Tacoma where he was declared dead.
About three hours later, officers found the truck abandoned about two miles from the shooting scene in thick brush off Sidney Road, but no one was inside.
Troopers, deputies and other officers searched the area for the driver using dogs and checking residences, Winger said. Dogs did not pick up a track, he said.
People who live in the area were urged to stay inside, lock their doors and call 911 if they saw anything suspicious.
Officers continued to question residents in the area where the truck was found. Three people who live nearby were later taken away for questioning at a separate location, officers said.
"It's obviously a high priority and drawing a lot of attention," said Sgt. Ken Dickinson. "We're looking for whoever is associated with that truck."
Investigators know who the registered owner of the truck is, but it's unknown if that person is a suspect in the shooting, Winger said.
"We know we have a registered owner, and that's all we know," he said.
Officers later towed the truck from the scene. Police also seized a white car found nearby that belongs to someone who may know the suspect.
Officers also said a house near the location where the truck was found has been raided before by state troopers during a drug bust.
Three schools in the area - Sidney Glen, Cedar Heights, and Mullenix Ridge - were placed on modified lockdown as a safety precaution. All are in the South Kitsap School District.
The fallen trooper's name has not been released pending notification of next of kin. Officials said he is a 44-year-old military veteran, who has an adult son now serving with the U.S. military forces.
The trooper had 16 years of service with the Washington State Patrol, working out of Bremerton. He was well-respected among his colleagues and loved working with schoolchildren in the WSP's outreach program.
"He worked in the Kitsap area for quite some time," Winger said. "He was a co-worker of mine."
The last time the trooper was heard from was when he made the traffic stop and radioed in the truck's license number.
"He routinely called the stop - license and location," Winger said. "He failed to answer status checks."
The deputy who arrived found the wounded trooper outside his patrol car.
Dozens of state troopers gathered at St. Joseph Medical Center and a procession of police vehicles escorted the slain trooper's body from the hospital to the Pierce County Medical Examiner's Office.
State Patrol Chief John Batiste made a televised statement later Thursday morning: "As you can imagine, it's a very sad day for the State Patrol; it's a very sad day for law enforcement in this great state of ours and across this country," he said.
The last Washington State Patrol trooper killed on duty was James Saunders, 31, who was shot in 1999 during a traffic stop in Pasco. Nicolas S. Vasquez pleaded guilty to aggravated murder and was sentenced to life in prison without parole.