ROSEMOUNT — Authorities said a driver and her daughter were killed when an 800-pound (360 kilogram) boulder rolled off the back of a landscaping truck and struck their car in suburban St. Paul.

Sixty-seven-year-old Karen Christiansen and her 32-year-old daughter, Jena Christiansen, were killed in Rosemount Monday afternoon when the boulder dislodged from the truck and hit their Toyota Avalon. They both died at the scene.

Rosemount Police Chief Mitchell Scott said 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.