FORENSIC experts have been left mystified over how a four-year-old survivor remained undetected for hours by police in the French Alps massacre.

They said police arriving at a crime scene should do thorough checks to make sure it is safe and that all survivors are accounted for.

But officers took eight hours to discover the girl, who was hiding underneath her dead mother's skirt, because "they didn't want to disturb the crime scene".

Jim Fraser, professor of forensic science at Strathclyde University, said in Britain a survivor would not be left so a crime scene was not compromised.

Some believe the girl may have been missed because an officer "panicked" when confronted with the horror.

In Britain, a doctor would be called to certify death – in doing so, in this instance, that may have alerted police that the child was alive.

Mr Fraser said officers confronted with such a crime scene should check the victims for signs of life.

"The idea that a child remained motionless in this vehicle undetected by the kind of police search I would have expected doesn't really work for me," he said.