ROSEMOUNT (AP) -- Authorities say a boulder estimated at 800 pounds fell off the back of a truck in Rosemount and struck a car killing a mother and daughter.

Police are looking for the driver of the truck. Sixty-seven-year-old Karen Christiansen and her 32-year-old daughter, Jena Christiansen, were killed late Monday afternoon when their Toyota Avalon was hit by the boulder that had dislodged from a commercial truck. The two died at the scene.

Rosemount Police Chief Mitchell Scott says a 33-year-old Hastings man was arrested Wednesday after investigators used surveillance video to identify the truck, its owner and the driver.

The man was booked into the Dakota County Jail. No formal charges have been filed.