Six people have died after a dust storm fueled by wind gusts topping 60 miles per hour (96 kilometers per hour) caused a pileup Friday evening on Interstate 90 in Montana, authorities said.

Twenty-one vehicles crashed and Montana Highway Patrol Sgt. Jay Nelson said authorities believe the weather was the cause.

“It appears as though there was heavy winds, causing a dust storm with zero visibility,” he said.

While the highway patrol did not have an immediate count of the number of injuries, Nelson said additional ambulances had to be called in from Billings to help.

The incident happened 3 miles (5 kilometers) west of Hardin.

The dust storm's roots can be traced back several hours, when storms popped up in central southern Montana between 1 and 2 p.m. (1900 and 2000GMT) and slowly began moving east, according to 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. (0300GMT) Friday.

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