At least 15 people were killed and more than 50 injured yesterday in a collision involving 22 tractor-trailers and 53 other vehicles in dense fog on Interstate 75 about 35 miles northeast of Chattanooga, Tenn.

Authorities speculated that vehicles continued to pile into each other for 30 minutes in a fiery chain reaction that produced one of the state's worst traffic accidents.

Cecil Whaley, operations director for the Tennessee Emergency Management Agency in Nashville, said fog formed so quickly along the Hiwassee River that most of the vehicles appeared to have slammed into each other at about 65 mph, their drivers unaware of the fog's depth and what it concealed.

"The trucks and cars rolled into that fog bank, and then they rolled into each other," Whaley said. "We have no evidence of any braking. They hit each other at interstate speeds."

Wreckage was strewn over four miles along both sides of the highway, and at least 10 fires were ignited, he said. Two state police officers were overcome by fumes from a chemical truck that spilled peroxide at the scene, he said.

The injured, including several with serious burns, were taken by helicoptor to hospitals in nearby cities, including Athens, Cleveland, Chattanooga and Knoxville, authorities said.

"The fog was so bad you couldn't see 10 feet in front of you," Norma Young, spokeswoman for the police and fire departments in Charleston, Tenn., told Reuter news agency. "That's always a bad place for fog. It's down in the valley, and it all just settles there."

By afternoon, Young said, the sun was shining brightly. Only one lane was reported open last night.

People involved in the accident described hearing booms in the fog as tractor-trailers collided with each other and with cars.

Ralph Fisher, 43, a salesman from Cleveland, Tenn., said he was northbound when he hit the fog, noticed traffic slowing and pulled to the roadside. "After that, I started hearing bangs and booms from everywhere," he told the Associated Press. "Immediately after that, there was a truck on fire from across the road."

Authorities said they believe that an initial accident occurred about 9:15 a.m. EST in the southbound lanes and that cars jumping the median or people rubbernecking to see the southbound crash may have caused the second pileup in the northbound lanes. Then, authorities said, there was a ghastly chain reaction.

Mary Youness, 55, of Cleveland, and her family were next to a propane tanker truck and heard crashes all around in the fog, she said, but managed to reach the shoulder unscathed.

"We feel terribly lucky," she said. "This truck was on the other side of us, and a car was smashed between {it} and another truck. And the truck driver said that, if the car had not been between them -- a girl was killed in the car that was between -- if the other truck had hit him, this explosion of propane would have wiped everything out."

The National Transportation Safety Board in Washington said it was dispatching six experts in highway and hazardous-material investigations. Whaley said it would take officials a long time to determine what happened.

I-75 is the main highway between Knoxville and Chattanooga and a heavily traveled route between Florida and northern states.