Mother, daughter killed when 800-pound boulder fell off truck, smashed their car; driver in custody: Police Karen Christiansen, 67, and her daughter Jena Christiansen, 32, died Monday.

A mother and daughter died when a roughly 800-pound boulder fell off a truck and smashed into their car, and now that truck driver is in custody, according to police.

Karen Christiansen, 67, was driving with her daughter, Jena Christiansen, 32, in Rosemount, Minnesota, Monday afternoon when the massive boulder dislodged from the back of the truck and crashed into their 2002 Toyota Avalon, the Rosemount Police Department said.

The mother and daughter, both of Shoreview, Minnesota, died from blunt force head trauma, the Hennepin County Medical Examiner's office said.

"My heart is broken," Jena Christiansen's boyfriend, Ryan Kilian, told ABC affiliate KSTP in Minneapolis. "She is the most amazing person I have ever met."

After the crash, authorities turned to surveillance video to track down the commercial truck involved, according to KSTP.

On Wednesday, detectives identified the commercial vehicle driver and owner as Joe Czeck, 33, of Hastings, Minnesota, police said.

Czeck was arrested and declined to give a statement, police said Thursday. He was taken to the Dakota County Jail. He has not yet formally been charged, Rosemount Police Chief Mitchell Scott told ABC News Thursday.