The accident happened around 8:15 a.m. near the Smokey Point onramp. The Washington State Patrol identified the deputy as Martin Weinbaum, 48.
Weinbaum was merging onto southbound I-5 when he lost control of his motorcycle, trooper Keith Leary said. A witness reported seeing the motorcycle flip end over end, Leary said.
The deputy landed in the middle of the freeway.
He was listed in stable condition with minor injuries at Harborview Medical Center in Seattle, sheriff's office spokeswoman Shari Ireton said.
Traffic was backed up for miles.
"This is being investigated as a one-motorcycle, injury collision," Leary said. "No other vehicles were involved. The weather at the time was overcast and raining moderately."
Paramedics transported the officer to the Arlington Airport where he was flown to Harborview.
The sheriff's office Collision Investigation Unit was working to determine the cause of the crash, taking measurements at the accident scene with assistance from Washington State Patrol.
The deputy, who has worked patrol for the sheriff's office for five years, was on duty when the crash occurred, Leary said.
Three lanes of southbound I-5 were closed Wednesday morning. All lanes were open by 10:45 a.m.
The state Department of Transportation estimated that traffic was backed up for six miles more than two hours after the crash.
Drivers were detoured onto Smokey Point Boulevard, then to 116th Street NE or to Highway 9.
Investigators are asking for any witnesses or people who were in the area at the time or shortly before to call detective Sgt. Jerry Cooper at 360-805-1192 or detective Ed Collins at 360-805-1194.
Wednesday's accident was second in three days involving a Snohomish County sheriff's deputy.
A patrol officer was injured Monday afternoon when his patrol car rolled over near Mariner High School south of Everett.
The deputy had made a U-turn to follow a silver BMW that was being driven recklessly. The deputy lost control, and his car flipped.
That crash happened about 4:30 p.m. along Fourth Ave W., in front of the high school.
The BMW also wrecked shortly after the deputy did, and the driver ran off.
Firefighters cut the deputy from the patrol car and took him to Harborview Medical Center in Seattle. The deputy, a 12-year law enforcement veteran, was treated and released from a Seattle hospital.
Eric Stevick: 425-339-3446; firstname.lastname@example.org.
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