EDMONTON -- A family road trip through North Dakota ended tragically yesterday when a head-on collision killed an Edmonton father and his daughter.

The man's 43-year-old wife -- the 11-year-old child's mother -- was flown to hospital with serious internal injuries after their minivan collided with a transport truck on Interstate 29 in Grandin, N.D.

"It's just a sad day," said Sgt. Troy Hischer with the North Dakota Highway Patrol. "It was very horrific."

Hischer said the 8:05 a.m. crash happened after the minivan driver inexplicably crossed three southbound lanes, straight into the path of an oncoming transport truck.

The 61-year-old trucker was not injured.

The posted speed limit on the highway is 112 km/h but police don't believe speed or alcohol played a role in the crash.

Police are now working to determine what caused the 43-year-old minivan driver to swerve on a clear, dry day. The road was straight, wide and recently paved.

"At this stage (it appears to be) some type of driver error," Hischer said.

"There wasn't a flat tire. We think it was some type of driver error: fatigue, maybe he reached for something.... But that would just be speculation."

Hischer said the family was returning from a trip through various parts of Canada and the U.S. They drove through Toronto and Winnipeg before crossing the border, he said.

On the day of the crash, the family was driving from Grand Forks to Edmonton, police said.

Crews arrived to find a demolished van, clothes strewn across the highway and a shaken trucker.

Finding a lifeless girl inside the wreckage hit rescuers hard.

"Most of us have children," said Hischer. "We deal with fatalities a lot but it really hits home when you see a child."

The victims' names are expected to be released this morning.

MICHELLE.THOMPSON@SUNMEDIA.CA