Two children are among the six people who died in a Montana pileup after a Friday evening dust storm caused blackout conditions on Interstate 90, a major route in both Montana and the Western U.S.

Montana Highway Patrol Sgt. Jay Nelson said investigators so far have found no other factors that contributed to the pileup that also sent eight other injured people to hospitals.

“Everything is indicative of an isolated extreme weather event,” Nelson said of the investigation, calling the crash among the worst he’d seen in 24 years with the state. “What could people do? It really was just panic.”

The pileup was just west of Hardin, with additional ambulances called in from Billings to help. The identities of the dead and conditions of the survivors are not yet being released.

The crash was reported around 4:30 p.m., as 21 vehicles, including six commercial semi-trucks, lost control in the dust storm that was fueled by gusts topping 60 mph (97 kph), authorities said.

Nelson said there was zero visibility for a mile-long stretch during a peak summer traffic hour for those commuting home from work or traveling for outdoor recreation.

It took more than six hours to fully reopen the road.

“We had a lot of debris and complete chaos,” Nelson said.

Gov. Greg Gianforte said on Twitter: “I’m deeply saddened by the news of a mass casualty crash near Hardin. Please join me in prayer to lift up the victims and their loved ones. We’re grateful to our first responders for their service.”

Montana Attorney General Austin Knudsen said in a statement that the Montana Highway Patrol, which he oversees, was investigating. “We will release more information as it becomes available and is appropriate out of respect of the lives lost and their loved ones.

A video from The Billings Gazette showed hundreds of tractor-trailers, campers and cars backed up for miles along the two eastbound lanes of the interstate.

Before the pileup, storms popped up in central southern Montana between 1 and 2 p.m. and slowly began moving east, said Nick Vertz, a National Weather Service meteorologist in Billings.

Those storms prompted a severe thunderstorm watch that covered Hardin and other parts of Montana from mid-afternoon until 9 p.m. Friday. Meteorologists forecasted the potential for isolated hail the size of a quarter, scattered gusts up to 75 mph (121 kph) and frequent lightning.

A so-called outflow — or a surge of wind that’s produced by storms — flew about 30 miles (48 kilometers) ahead of the storms, Vertz said.

Winds picked up quickly around the time of the crash, according to readings at nearby Big Horn County Airport. A 40 mph gust (64 kph) was recorded about 15 minutes before the crash was reported and in less than an hour another burst of wind hit 64 (103 kph).

The wind easily picked up dust — a product of recent temperatures into the 90s and triple digits over the last week — and reduced visibility to less than 1/4 mile (0.4 kilometers).

“If they looked up in the sky while they’re in Hardin, they probably didn’t see much of what you’d think of for a thunderstorm cloud, maybe not even much at all,” Vertz said. “It was just a surge of wind that kind of appeared out of nowhere.”