BOULDER, Colo. -- A female bicyclist was struck and killed in front of her mother in a crash that involved two semi trailers and two other vehicles, the Colorado State Patrol said.

The accident occurred at about 10 a.m. Monday on the railroad tracks at 63rd Street and the Diagonal Highway in Boulder County.

State troopers said a mother and her daughter were riding their bicycles south on 63rd Street. As they crossed the Diagonal Highway, a gravel truck hit a semitrailer that was then pushed into the daughter's bicycle.

The woman killed was identified as 39-year-old Linnea Luscher of Nederland.

Troopers believe the gravel truck ran a red light. They are focusing their investigation on the gravel truck to see if the brakes failed or if there was some other malfunction.

Troopers say Luscher's mother was not injured, but the drivers of both the gravel truck and the big rig were taken to local hospitals with non-life-threatening injuries.

One driver stopped to helped Luscher, but said the accident was just too horrific.

"There was absolutely nothing we could do. Well, what we did do was prayed, we stood over her and prayed. That's all we could do," said witness Therese Simpson, crying.

"There's not a whole lot the bicyclists could do. They probably didn't even see it coming," said witness Dan Brillon. "It's a busy highway, but there's also a lot of bike traffic. I think generally, most motorists are aware of the bikes."

Concrete Express Inc., the owner of the truck believed to be responsible for the accident, would not comment specifically about the accident, but the company's attorney said they are working with authorities and express their sympathies to the families involved.