At least six dead - including two children - after dust storm triggers mass pileup in Montana

Two children are among the six people reported dead after a dust storm fueled by wind gusts topping 60mph caused a pileup on the Interstate 90 freeway in Montana late Friday, authorities said.

Investigators so far have found no other factors beyond the dust storm that contributed to the pileup which also sent eight injured people to hospitals.

More than 20 vehicles were involved in the crash. Sgt Jay Nelson of Montana highway patrol said, "Everything is indicative of an isolated extreme weather event. What could people do? It really was just panic."

Nelson said the crash was among the worst he'd seen in 24 years with the state.

Nelson also said additional ambulances had to be called in from Billings to help Friday.

The governor, 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."

The incident happened 3 miles (5km) west of Hardin and involved 21 vehicles, among them six commercial semi-trucks. 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, Nelson said.

It took more than six hours after the wreck to fully open the road.

Nelson added: "We had a lot of debris and complete chaos."

The dust storm's roots could be traced back several hours, when storms popped up in central southern Montana between 1pm and 2pm and slowly began moving east, according to Nick Vertz, a National Weather Service meteorologist in Billings.

A so-called "outflow" or a surge of wind that is produced by storms but can travel faster than them flew east-south-east about 30 miles ahead of the storms, Vertz said.

A 40mph (64km/h) gust of wind was recorded at the nearby Big Horn County airport at 4.15pm. The crash was reported to the highway patrol at 4.28pm.

By the airport weather station's next reading at 4.35pm, the gusts had picked up to 62mph. Another reading 20 minutes later recorded a gust of 64mph.

The wind easily picked up dust - a product of recent temperatures into the 90s Fahrenheit (30s Celsius) and triple digits over the past week - and reduced visibility to less than a quarter mile (0.4km).

"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."

As first responders attempt to clear the wreckage, the meteorologist said they could expect to be safe from additional winds and thunderstorm activity.

"It should be a relatively clear, calm night for them."