DIVERNON, Ill. (AP) — Winds stirred up a wall of dust from farm fields on May 1 that engulfed a stretch of busy I-55 in a matter of minutes. The brown cloud’s intensity caked even the insides of vehicles in dirt. As darkness enveloped them, some cars and trucks hurtling down the road put on their brakes; others didn’t.

They slammed into one another, leaving them mangled or in some cases burned. And when it was over, almost 40 people were injured and seven people were dead - at least two of them still unidentifiable.

“They were very unusual circumstances. Certainly dust storms happen, but it is not something that happens every day here in this part of Illinois or any part of Illinois,” Illinois State Police Director Brendan Kelly said at a news conference.

The highway was closed in both directions after the late-morning crashes on May 1, a scene Gov. J.B. Pritzker described as “horrific.” Northbound and southbound lanes reopened around 6 a.m. Tuesday, May 2 Kelly said, before state police closed the same 20-mile section of road again late Tuesday afternoon as high winds continued. The traffic advisory indicated there had been no accidents but the action was taken as a precaution. The section reopened later Tuesday afternoon.

Earlier, Kelly had said it was extremely rare to close a highway due to weather except in blizzard conditions. Noting three memorable dust storms in the same region since 1983, the National Weather Service reported that one, in 1990, resulted in closure of an 18-mile section of Interstate 57.

The May 1 crashes involved 40 to 60 cars, along with tractor-trailers, two of which caught fire, state police said. The six people who died were all in northbound lanes, while 37 people on both sides of I-55 were taken to hospitals.

Witnesses described a sudden burst of dirt completely erasing visibility - “It was like a white out, only it was a brown out,” said Evan Anderson, 25, who was returning home to St. Louis from Chicago.

Caught between two semis, Anderson said he believes he was spared serious injury because the truck behind him turned slightly before striking his vehicle.

“People tried to slow down, and other people didn’t, and I just got plowed into, ‘’ Anderson said. ‘’There were just so many cars and semitrucks with so much momentum behind them.”

Winds were gusting between 35 and 45 mph, the National Weather Service said. Meteorologist Chuck Schaffer said the area where the crashes occurred is ‘’very flat, very few trees.”

Farmers in central Illinois, including Montgomery County, where the crashes occurred, are tilling fields and planting corn and soybeans, the region’s chief crops, said Emerson Nafziger, a professor emeritus in the Department of Crop Sciences at the University of Illinois’ Urbana-Champaign campus.

Much of central Illinois has received little rain in recent weeks, he said, and cropland that is normally wet this time of the year is dry - and with farmers active in their fields, high winds can easily send dust airborne.

“It just has to dry the top surface, a quarter-inch of soil, and then there’s a huge amount to blow around,’’ Nafziger said. ‘’In this case, a lot of fields have been tilled, some have been planted, but the tillage process and even spreading fertilizer will put up a fair amount of dust.”

Kelly said his investigators would try to determine whether nature was the only factor.