ROSEMOUNT, Minn. – An 800-pound boulder rolled off of a landscaping truck and struck a car in Minnesota, killing a mother and daughter, police say.

Karen J. Christiansen, 67, and Jena Christiansen, 32, both of Shoreview, were pronounced dead at the scene Monday, police say.

Rosemount Police arrested the driver of the landscaping truck Wednesday. Police Chief Mitchell Scott said the 33-year-old Hastings man is being held in the Dakota County Jail. No formal charges have been filed.

More: Man wasn’t drinking while driving, just sipping at stop signs, he told deputies

The crash occurred Monday afternoon in Rosemount, a suburb of St. Paul. Scott said the boulder likely dislodged from the truck as it drove over a railroad crossing. The rock then bounced and rolled into oncoming traffic, colliding with a Toyota Avalon.

"The boulder is approximately 800 pounds," Scott said. "It went through the vehicle. It went through the windshield and went completely through the vehicle."

Scott said witnesses heard the rock hit the road and then strike the vehicle. Police are still investigating why the driver of the landscaping truck didn't stop.

On Wednesday, authorities used surveillance video were able to identify the company that owned the commercial vehicle and the driver.