A boulder estimated to weigh 800 pounds fell off the back of a truck in Rosemount and struck a car going in the opposite direction, killing a mother and daughter from Shoreview.

Rosemount police are looking for the truck and its driver, Police Chief Mitchell Scott said Wednesday. He called accidents like this one extraordinarily rare.

"You could try to recreate that a million-plus times and you could never recreate that," he said.

Scott said several investigators have been working on the case since the women were killed late Monday afternoon.

Police said a 2002 Toyota Avalon was traveling north on Rich Valley Boulevard south of 125th Street when the boulder fell off the commercial truck, which was headed south. Scott said the boulder was airborne when it hit the vehicle.

The driver of the car, Karen J. Christiansen, 67, and her passenger, Jena H. Christiansen, 32, died at the scene.

Scott estimated the boulder was about 3 feet by 3 feet.

Jena's boyfriend, Ryan Kilian, said Wednesday night that his heart is broken.

"The world lost an angel," he said, describing Jena as someone who would do anything for anyone who needed help.

Kilian said he knew it was an accident, but said her death ruined many lives, including his.

Jena was the general manager at several Red Lobster locations. "All of her employees loved her," he wrote in a message.