Dense fog and smoke from two marsh fires collided early Monday to create a "superfog" that blanketed Interstate 55, causing a mile of deadly, chain-reaction crashes that killed seven motorists and shut down the highway in both directions, authorities said.

At least 158 vehicles were involved in the pileups and the search for victims was ongoing late Monday, according to Louisiana State Police. Twenty five people were transported to area hospitals with injuries, and many others sought medical assistance on their own, state police said.

Fires also broke out along a portion of the crash scene, and police were attempting to move a tanker truck carrying a hazardous liquid to better assess vehicles in that area. More fatalities were possible, they said.

"We’re grateful because we’re alive," said Clarencia Patterson Reed, 46, who witnessed at least two people perish when their box truck crashed into a flat-bed trailer. "We’re here to tell you what we see up here because some people will not get that chance. It's bad."