HARDIN, MT (Associated Press) — Authorities say six people have died after a pileup Friday evening on Interstate 90 in Montana.

Twenty-one vehicles crashed and Montana Highway Patrol Sgt. Jay Nelson said authorities believe the weather is to blame. He said it appears heavy winds caused a dust storm with zero visibility.

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.

Gov. Greg Gianforte said on Twitter that he was deeply saddened by the news of a mass casualty crash.

The incident happened just west of Hardin.