RINGGOLD, Ga. -- About 125 vehicles piled up in a foggy chain-reaction crash that killed four people and left a half-mile trail of mangled cars and trucks on a highway in northwest Georgia Thursday.

The accident, about five miles south of the Tennessee line on Interstate 75, shut down the highway in both directions and caused 39 injuries, 15 of which were described as serious.

The crash occurred in a thick fog during rush hour, around 7:50 a.m. By Thursday afternoon, dozens of dented and smashed vehicles still littered the road and two bodies remained stuck in crushed cars.

Smaller vehicles crushed

Vehicles on both sides of the highway were involved in the accident, which included at least two large pileups. Two separate crashes occurred in the northbound lanes and two others in the southbound lanes, all because drivers were going too fast for conditions, a spokesman with Georgia State Patrol said Thursday afternoon.

"Weather conditions and excessive speed obviously became a deadly combination," GSP spokesman Jim Shuler said.

All the fatalities were in the northbound lanes, where a tractor-trailer ended up on its side. It was unclear what made the truck go out of control.

The trailer crushed several vehicles and dozens of other trucks and cars piled into the wreckage.

"I heard a big truck screeching," said Debbie Grant, who lives across from the highway. "I heard a crash. I heard another crash and then another crash. I just heard crash after crash. I heard people screaming."

Jeffrey Woods, 38, of Knoxville, Tenn., said he was three or four vehicles behind the initial accident and ran into a "pillar" of fog. He said his SUV ricocheted off a tractor-trailer and under another 18-wheeler.

"I had a bunch of blood flowing from my head, then I realized I need to turn the engine off," Woods told the Huntsville (Ala.) Times. "I crawled through the window. I was concerned about anything exploding. I could smell diesel. I could hear vehicles banging behind me."

About 300 emergency workers responded. The walking injured were taken to a fire station by school bus.

Many of the vehicles involved were still entangled by midday, rescue workers continued to swarm the scene and I-75 was deserted for miles in either direction. At least 20 wrecked tractor-trailers were left, some of them on top of cars.

Catoosa County Sheriff Phil Summers said all the dead had been identified, but their names were withheld pending notification of their families.

Summers said no charges would be filed.

"It did not appear that there was any fault other than fog," he said. "There is no plan of prosecution. This was an act of nature."

The highway does not have fog lights, and the area -- unlike the mountains of Tennessee north of Ringgold -- rarely has such thick fog, Summers said.

Each morning, thousands of commuters who live in northwest Georgia head north to work in and around Chattanooga.

Roger Lee of LaFayette was surprised when he drove into the heavy fog on his way to work in Chattanooga.

"It was patchy," he said. "When I first got on the highway there was no fog."

His pickup slammed into the large pileup in the northbound lanes and he had a sprained arm and a black eye.

"You're on the interstate; you don't ever imagine that everything is going to stop," he said.

Rhonda Frix, an employee at Catoosa Utility in Ringgold, said she avoided I-75 Thursday, partly because the fog was so bad.

"It was almost zero visibility out there," Frix said. "It was worse this morning than it has been in a long time."

The Georgia State Patrol opened two southbound lanes Thursday afternoon, but the northbound lanes would likely be closed until early Friday.

The crash happened about 40 miles from the site of a deadly 1990 pileup near Calhoun, Tenn., that involved 99 vehicles and left 12 people dead. That accident prompted Tennessee officials to install an elaborate fog detection and warning system on that fog-prone stretch of I-75 near the Hiwassee River.