Troopers said trouble began when the truck traveling north veered off the road and slid onto a roadside embankment at a high rate of speed. The truck flew forward and smashed into an overpass at 24th around 6:30 p.m.

The impact sheared off the top of the vehicle's cab and ripped off the tank, releasing the chemical stored inside.

Troopers initially identified the spilled chemical as liquid nitrogen and took extreme caution, but later redefined it as carbon dioxide used to carbonate sodas.

"We didn't even go in and get the driver," said Sgt. Sam Ramirez of the initial response. "we didn't know what we had. We did a worst-case scenario (response), evacuated about a quarter mile in every direction."

Just before 9 p.m., the southbound lanes reopened, and the northbound lanes followed suit about a half hour later.

Troopers said the truck driver, A 46-year-old Auburn man, was taken to Harborview Medical Center, where he is listed in critical condition. It is not known what had caused him to veer off the road.

The overpass bridge at 24th appears to be significantly damaged, but it is still safe to drive under it, Kessler said. The bridge, which sustained a foot and a half-long indentation from the crash, will remain shut down until the Department of Transportation can assess its safety.

"And to have one that's reinforced concrete and rebar that's like wire strands in there...to see all of that exposed, I can tell you that it was a heck of a hit," said Ramirez.