The Washington State Patrol says a semi with two FedEx trailers jackknifed on wet pavement about 4:30 a.m. One trailer crushed the cab against a guard rail while the other trailer tipped.
The cab and one trailer caught fire when a fuel line was ripped open. It was extinguished by firefighters with foam.
The driver was shaken but unhurt.
The crash blocked southbound lanes of the freeway between the state Convention Center and I-90 until about 9 a.m., when two lanes were reopened. All lanes were later reopened, but traffic did not return to normal until early afternoon.
Another backup formed earlier at the interchange of I-405 and I-5 near Lynnwood, as motorists tried to detour around the crash scene, said Mike Allende of the state Department of Transportation.
More accidents occurred earlier in the backup. All of those were cleared.
The cleanup was complicated by the fact that the semi truck cab was lodged beneath one of the trailers, which required a carefully coordinated towing operation.
Diesel also began spilling from the crashed truck during the operation, causing further delays.