CONEMAUGH TOWNSHIP, Pa. -- A tree came crashing down on top of a car that was driving down a highway in southwestern Pennsylvania, killing two young sisters who were riding in the back seat, CBS Pittsburgh reported Tuesday.

Four other people, including two more small children and a pregnant woman, were also injured in the accident Monday in Somerset County.

Police said the incident, which happened around 2:30 p.m. along Route 403 in Conemaugh Township, was a freak accident. Conemaugh Township Police Chief Louis Barclay told The Tribune Review that dry rot apparently caused the tree to topple.

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Police responding to the scene found the male driver unconscious inside the Pontiac Grand AM. A pregnant female passenger had managed to get out of the car.

"I saw this lady get out of the passenger seat and she was yelling, 'My babies,'" one witness told CBS Pittsburgh. "We did watch them cut the doors off and cut the roof off, and get the driver out of the car."

Authorities said two little girls who were riding in the backseat died of blunt force trauma. They were identified as 8-year-old Ryleigh Freiwald and 6-year-old Macayla Freiwald.

Two other girls, 5-year-old Ciara Freiwald and 3-year-old Lillie Freiwald, were injured in the crash.

The four girls' mother, 26-year-old Ashley Lichty, was taken to Conemaugh Memorial Medical Center in Johnstown, Conemaugh Township Police Chief Louis Barclay told The Tribune Review. Barclay said Lichty was eight months pregnant and had not lost the child.

The driver, Jason Himebaugh, 36, was also being treated at the hospital.

Himebaugh has been dating Lichty for about 18 months and is the father of her unborn child, his friend Dan Gorgone told the newspaper.

Paramedic Adam Trabold told The Tribune Review it was very traumatic for first responders.

"In the 10 years I've been here, this is a 9 out of a 10 on a scale," he said.