RIVERDALE — A man and his 5-year-old daughter died, and the mother and another daughter were critically injured when the car they were in was pushed into the rear of a semi truck Sunday.

At 5:23 p.m., Utah Highway Patrol troopers were performing a "slow down," where a trooper with lights and sirens drives back and forth across all lanes of traffic to hold traffic back and create a safety zone for another trooper to remove a hazard from the road.

During the procedure, a Mazda passenger car headed north on I-15 near the I-84 junction was stopped behind a semi truck when a Chevy passenger car driving at freeway speeds collided with the rear of the Mazda, pushing it into the rear of the semi truck, according to UHP Sgt. Mary Kay Lucas.

The Mazda had four occupants: A 30-year-old man was pronounced dead at the scene. His wife, who is 28, was taken to a hospital in extremely critical condition and has since been upgraded to serious condition, according to Lucas.

Two children were taken via helicopter to a local hospital, where the 5-year-old girl was pronounced dead and a 7-year-old girl remained in critical condition Sunday night, according to Lucas.

"They're in really bad shape," Lucas said.

A dog inside the Mazda was also killed in the crash, she said.

The victims' names were not released Sunday evening. It was unclear whether any of the victims were wearing seatbelts, and an official cause of the accident hadn't been determined. The drivers of the Chevy and the semi truck were not injured.

Northbound I-15 was closed at Riverdale Road while officers investigated the crash and later reopened. No citations had been issued Sunday evening, though the investigation was still ongoing, the sergeant said.

It was the second deadly car crash on Sunday involving a stopped vehicle on the freeway, according to Lucas.

"People just need to be a little more vigilant and aware when they're traveling that sometimes vehicles will stop suddenly in front of them or be stopped, and they need to be paying constant attention to what's going on around them," she said.

Email: mjacobsen@deseretnews.com, Twitter: MorganEJacobsen