Details about the shooting are sketchy, but at least one motorist called police at around 7 p.m. to say a man in a truck had swerved and blocked two lanes of southbound traffic on I-5 on the Ship Canal Bridge.
The callers said the man got out of the truck and began spray painting a green circle on the roadway. At some point the truck caught fire, according to Washington State Patrol Troopers.
Police later said they found "incendiary devices" in the man's truck.
Two troopers responded to the scene and attempted to use a Taser on the man, who by that point was displaying a knife, according to WSP officials.
It's unclear exactly what happened next, but trooper Chris Webb said moments later a trooper opened fire on the man, hitting him at least once.
"You've got several cars, several witness still around this subject, who is obviously acting erratically, and we don't know exactly why," said trooper Chris Pratt.
Neither trooper was injured.
Medics arrived soon after and transported the shooting victim, a man in his 20s, to Harborview Medical Center for treatment, according to Seattle Fire Department officials.
His condition is not known.
All southbound lanes of I-5 were immediately shut down at the bridge, causing a massive traffic backup that spanned miles.
Drivers stuck in the southbound lanes near the bridge were turned around and allowed to exit onto 45th Street, according to the Washington State Department of Transportation. Police say they wouldn't be surprised if all southbound lanes remained closed past midnight.
Seattle Police and WSP are handling the investigation. SPD's Arson Bomb Squad was also deployed to help with the investigation.