Six people died Friday in a buildup of highways in Montana, possibly caused by a dust storm, authorities said.

A series of collisions involving 21 vehicles occurred on Interstate 90 near Hardin, Montana Highway Patrol Sgt. Jay M. Nelson said by email.

“A fast-rising dust storm caused a 21-vehicle incident tonight,” he said.

Officials did not immediately release information about possible injuries Friday night.

Delays on the highway are expected to last for hours, Nelson said.

All eastbound traffic has been diverted from the freeway and westbound traffic has been reduced to one lane, according to the Montana Department of Transportation, which first reported the buildup near mile post 493 at 4:55 p.m.

“I am deeply saddened by the news of a mass casualty crash near Hardin,” Governor Greg Gianforte tweeted. “Please join me in prayer to lift the victims and their loved ones. We are grateful to our first responders for their services.”

The National Weather Service had issued severe thunderstorm warnings and warnings for southern central and southeastern Montana Friday. It warned that gusts of up to 55 mph were possible all night.