The two women died at the scene, WCCO reported.
"You're required by law to have whatever you are hauling secured in your vehicle," Rosemount police Chief Mitchell Scott told WCCO. "So if you have a lawn mower or an ATV (all-terrain vehicle), UTV (utility vehicle) inside, you are responsible to make sure it's secure."
Surveillance video from a business near the incident showed a truck carrying large boulders. After the truck crossed some railroad tracks, one unsecured boulder flew out of the truck, crossed the center line of the road and crashed through the windshield of Christiansen's vehicle, KSTP reported.
According to court documents, the truck continued after the boulder fell. The driver then pulled over and parked in a driveway. The video showed a person walking to the back of the truck, KSTP reported.
A witness who drove past the parked truck told authorities of seeing a man outside the vehicle strapping large boulders down, KSTP reported.