Two children are among the six people who were killed in a pile-up after a dust storm caused blackout conditions on Interstate 90 in the US state of Montana.

Montana Highway Patrol Sgt Jay Nelson said investigators so far have found no other factors that contributed to the crash that led to another eight people being taken to hospital.

“Everything is indicative of an isolated extreme weather event,” Sgt 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 pile-up took place just west of Hardin, with additional ambulances called in from Billings to help. Interstate 90 is a major road in both Montana and the western US.

The crash was reported around 4.30pm on Friday afternoon (11.30pm BST), as 21 vehicles, including six commercial semi-trucks, lost control in the dust storm that was fuelled by gusts topping 60mph.

Sgt 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.

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.”

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.

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

The wind easily picked up dust – a product of recent high temperatures – and reduced visibility to less than quarter of a mile.