Walker was taken into custody at his home in Marysville where he had lived for several months, neighbors said.
The Sheriff's Office will hold a press conference at 11:30 a.m. Monday at the Snohomish County Courthouse to answer questions about the case.
The investigation is ongoing, Sheriff's spokeswoman Shari Ireton said.
Walker was booked on suspicion of first-degree murder and multiple firearms counts late Friday night and was lodged in the Snohomish County Jail.
Residents of the Marysville neighborhood where he was arrested said that heavily-armed Snohomish County Sheriff's deputies raided the home on 58th Drive N.E. Friday to arrest Walker, who initially tried to drive away in his car.
"We hear noises down at the end of the street yelling get down on the ground, get down on the ground now," Jennifer Redding said.
Walker apparently worked in information technology at Boeing, studied computer network systems at ITT Tech in Everett, and graduated from Stanwood High School, according to his Facebook page.
His page also features a top photo displaying various firearms.
Conley was out walking with friends on a road near her friend's house in the 10600 block of South Lake Stevens Road when she was shot in the neck and died.