Police say the death of a two-year-old Calgary girl whose head was caught in the power window of a running SUV is a "tragic accident."

Investigators say the toddler died Tuesday morning after her mother left the girl and her six-year-old brother in a Chevrolet Blazer while she ran an errand.

The mother had taken the day off work to prepare for the little girl's birthday, which would have been Wednesday.

"We've investigated (and) we feel strongly from witnesses that it was misadventure," Calgary police Duty Insp. Luch Berti said.

"It was the child's own actions that caused the window to go down and then back up so unfortunately it's just one of those tragic accidents that we just can't explain," Berti told CTV Calgary.

Police say the incident took place when the girl's mother stopped to run a quick errand on the way to dropping her son off at school.

She left her children in the vehicle with the engine running.

While she was away from the SUV, the older child unbuckled his two-year-old sister because she was crying. But he fell asleep soon after.

Officials believe the toddler activated the power window with her knee as she looked out the window, becoming stuck.

"She had propped her legs up on the arm-rest and activated the window to close on her neck," said Det. Andrew MacLeod.

A passerby walked up to the vehicle when he heard the girl calling for her mother, realized the girl was stuck and managed to free her.

He placed her on the car seat and then ran into a building to call emergency services.

Meanwhile, the mother returned to the car, assuming both her children were sleeping, buckled her daughter into the car seat and drove away.

It wasn't until about 40 minutes later that noticed her daughter was unresponsive and also called 911. A police cruiser driving past pulled over and two officers helped the woman, performing emergency CPR on the child.

"To step out of your regular duty, to do something that, really, none of us should ever have to do, is exception," said Ben Morgan of the Calgary EMS.

Emergency crews soon arrived and an ambulance transported the girl under police escort to hospital, where she was pronounced dead.

Officials say the tragic death serves as an unfortunate reminder that children should never be left unattended in a vehicle.

The issue has also raised questions on whether power windows should be made child-proof, and whether it should be illegal to leave children alone in cars.

"In the United States, numerous states have laws that make it a traffic violation to have children unattended in vehicles. Canada doesn't have such a law," said Tracey Warren of Child Safe Canada.

With a report from CTV Calgary's Camilla Di Giuseppe