Police shut southbound Interstate 5 in Seattle for hours on Monday after shooting a driver who had stopped to begin spray-painting the roadway and then brandished a knife when confronted, officials said.

Police received calls at about 7 p.m. (0200 GMT) that a pickup truck had swerved, blocked two lanes and then caught fire, said Washington State Patrol spokesman Cliff Pratt. State troopers rushed to the scene and as one of them and an accompanying transportation official sought to put out the blaze, another trooper approached the driver.

"Prior to that, 911 calls had this driver with some type of aerosol can spraying the freeway," Pratt said. "And just looking back there, for some reason..., there is a very large green circle right in the middle of the freeway. Have no idea why."

The suspect, a man in his 20s, took out a knife when a state trooper confronted him, Pratt said. A trooper then used a stun gun to try to subdue the man but it "had no effect", Pratt said, and police then shot and wounded him.

The man was taken to Harborview Medical Center in Seattle. His condition was not immediately known.

An investigation was under way, Pratt said. The incident backed up traffic for miles on Interstate 5, the main interstate highway running along the U.S. West Coast.

(Reporting by Eric M. Johnson in Seattle, editing by Mark Heinrich)