TACOMA, Wash. - Investigators say it could take days to determine what caused a propane tanker truck to explode, igniting a massive fireball that shut down a highway, cut power to thousands and injuring four.

The initial explosion happened while a delivery truck loaded with about 8,000 gallons of propane exploded at Atlas Castings and Technology near the intersection of Interstate 5 and State Highway 16 around 3 p.m. Saturday, State Patrol Trooper Brady Kessler said.

The truck driver, identified as Charles McDonald, 64, remained in critical condition Sunday at Harborview Medical Center in Seattle. Three men who were taken to St. Joseph Medical Center in Tacoma were treated for minor injuries and released.

An Atlas executive said the company had accounted for all 32 employees who were working at the plant on Saturday.

A large two-axle portion of the tanker truck blew more than 150 feet into the air and landed in flames on the Highway 16 overpass, along with parts of the truck cab and other debris, state Department of Transportation workers told The News Tribune in Tacoma. No cars were struck.

Highway 16 was reopened Sunday morning. Transportation Department engineers had to wait until until fire officials determined they could safely inspect bridge piers beneath a stretch of State Route 16.

Blast heard for miles

The explosion heard miles away sent a ball of fire over the historic Atlas Foundry Saturday afternoon, shutting down the highway and cutting power to the city's industrial area. The first blast was reported at roughly 3 p.m. and another followed.

Power was completely restored to all customers by about 1 p.m. Sunday.

The Federal Aviation Administration established a five-mile-perimeter no-fly zone around the foundry on Saturday. The restriction was lifted around 3 a.m. Sunday, after authorities determined the site no longer posed a safety threat to aircraft, FAA spokesman Mike Fergus said.

Through the night, fire crews sprayed down two propane tanks to keep them from exploding as well.

The incident started when a tanker truck was off-loading fuel. Jeff Merrill, of Washington State Patrol, said the tanker may have leaked and sparks from nearby welding may have ignited fumes.

The foundry is located at 3021 South Wilkeson in Tacoma.

A large two-axle portion of the tanker truck blew more than 150 feet into the air and landed in flames on the Highway 16 overpass, along with parts of the truck cab and other debris, state Department of Transportation workers told The News Tribune in Tacoma. No cars were struck.

"We're fortunate it didn't land on any car ... that just tells you how violent of an explosion this was," Merrill said. "It's anticipated we could find truck and tractor debris in a mini block radius."

Tacoma Deputy Fire Marshal Kevin O'Donnal said 50 firefighters had the two-alarm fire under control by Saturday evening. He said an electrical substation also exploded.

"You could feel the heat like you were right there in front of it," said Sam Jackson, who was in the area. "It felt like an atomic bomb or something pretty heavy."

The tanker driver was airlfited to Harborview, where he remained in critical condition Sunday. The three other injured patients had non life-threatening injuries and were treated and released. An executive of the company said all employees had been accounted for.

Joe Farmer, supervisor of the cleaning room at Atlas, was nearby when the first explosion occurred.

Photo by Matthew Barnes

The blast at Atlas Foundry was heard at least two miles away and caused lights to flicker.

"It blew me off the forklift, Farmer said. "It blew out several windows. All of the buildings nearby used to have junk all over the tops of them. It's not there anymore."

Farmer estimated that between 30 and 40 people were in the foundry on Saturday.

The foundry is near the intersection of I-5 and Highway 16 in the "Nalley Valley."

The eastbound lanes of State Route 16 remained closed into the evening as state transportation officials waited for a safe time to send bridge inspectors to check a highway overpass.

An electrical substation was damaged in the blast, knocking out electricity to about 13,000 Tacoma Power customers, said utility spokeswoman Chris Gleason. All but 200 were back within an hour. The remaining customers could be without power for a while, Gleason added.

The Tacoma News Tribune reported that explosions sent flames two stories high, and a "fireball" 1,000 feet tall.

Due to the concern about the possibility of addtional explosions, five nautical miles and 10,000 feet of airspace were closed down in the area. Sea-Tac spokesperson Perry Cooper said according to his records, no flights were affected because of air space closure

KING

The fire burned for hours after the explosion.

Caller Dave Damron told KING 5 he was getting out of his car when two explosions "rocked the neighborhood."

"I see a flaming gas jet, probably 25 to 30 feet in the air," he said.

Witness Katie Martin said she was stopped at S. 38th Street Exit off I-5 when it happened.

"All of the sudden 'boom,' this huge explosion, a huge mushroom of fire and it got so hot that I could feel it through my windshield," she said.

Alan Bouman, a worker at T & T Tire Factory a couple of miles away, said the blast caused lights to flicker and a door to sway open.

"Some of the guys outside asked if you heard that, heard the explosion," Bouman said.

Keith Williams, an employee at the foundry, said people were working at the plant today. He says workers were there weld casting for ships.

According to it's Web site, "Atlas Castings & Technology is one of the premier foundries in North America producing high integrity steel castings for customers around the world. Atlas uses state of the art technology to produce complicated components with stringent metallurgical and quality requirements. Atlas is no longer just a foundry and has evolved into a manufacturer of major metal components."