One sister is a "key witness" still under guard in a French hospital, her health too fragile for questioning. The other escaped the gunman who killed their parents by hiding beneath her dead mother's skirts, but can't explain what happened during their French Alps vacation. And, experts say, both could still provide crucial help — if handled carefully.

The young British sisters, who survived the mysterious shootings a week ago, are the only known witnesses to the crime. And experts say 4-year-old Zeena and 7-year-old Zaina could be the key to solving the case.

But it's a delicate task. There are plenty of examples of investigators botching interviews with child witnesses, sometimes with devastating results.

"Children that age are capable of providing tremendously accurate long-term memories," said Stephen Ceci, a professor of developmental psychology at Cornell University who has studied child witnesses. However, he said, children's memories are susceptible to corruption if interviews go badly at the outset of an investigation.

The most important thing, he said, is to develop a rapport and ask open questions. "You resist the impulse to start providing cues."

In the case of the two British girls, Ceci said native English speakers would be vital.

"There are all these really nuanced aspects of a language," he said.

Eric Maillaud, the prosecutor leading the investigation in France, told The Associated Press that the younger girl described hearing a shout that might have come from an assailant, but could not specify a language. "What can you expect from a terrorized 4-year-old girl?" he added.

As for the older girl, Maillaud said she had reached "the age of reason" but was still unlikely to unravel the case. The girl will have a British interviewer and a translator when she is finally able to talk.

"When it comes to the probe we cannot base the outcome of the investigation on what a severely injured 7-year-old girl says," the prosecutor said Wednesday.

But Michael Lamb, a Cambridge University professor of psychology who specializes in child witnesses, said both girls could potentially help investigators. They could, he said, say what language was spoken, whether the car was stopped unexpectedly and whether they recognized the attacker.

What is key, Lamb emphasized, is letting a child speak freely, without interrupting or leading the conversation in a particular direction.

"We all think that it's really easy to talk to a young child, and the fact is, it actually isn't. It takes quite a lot of skill to conduct an interview in a way that simply provides a platform that encourages a child to keep talking."

Pitfalls abound. Many of the daycare sex abuse scandals of the 1980s and 1990s in the U.S. resulted in wrongful convictions and outlandish stories.

Among the most notable was the McMartin Preschool case in Southern California, with its bizarre tales of underground tunnels and satanic sacrifices. The trial — the most expensive in the U.S. — lasted seven years and involved hundreds of children, only to end in 1990 with no convictions on unreliable and, experts say, coerced testimony.

But even very young children who were crime victims or witnesses have been able to provide crucial help. Lamb said among the most famous was the 1983 case of a 3-year-old kidnapped from her Denver-area neighborhood. The girl was found by birdwatchers three days later, at the bottom of a mountain outhouse pit.

Her kidnapper had been detained just after she disappeared, then released. She picked him out of a photo lineup, telling police: "That one; he did it. He put me in the hole." Police issued an arrest warrant that day for Robert Thiret, who was later convicted in the kidnapping.

And in 2010, a Tennessee man named Jessie Dotson was convicted in the 2008 murders of his two young nephews, his brother, his brother's girlfriend and two family friends, in large part on the testimony of a nephew who was 9 the day of the attack. The boy, who survived a knife blade plunged into his skull, pointed to his uncle in the courtroom and said the killer was "right there."


John Heilprin in Annecy, France contributed to this report.