Questioning of the seven-year-old British girl considered a "key witness" in the French Alps murders will be carried out by investigators specialising in child testimony.
"The investigators want to speak to her as quickly as possible and with the greatest sensitivity possible," said prosecutor Eric Maillaud who has ruled out getting any information from Zainab al-Hilli's younger sister Zeena, four.
Zainab was beaten on the head and suffered a fractured skull as well as being shot in the shoulder during the attack on Wednesday.
Left for dead near the car where her parents and grandmother had been killed with two bullets to the head, she was found by a passing British cyclist.
For now, Zainab is in an induced coma in hospital.
But once she is conscious again, and when doctors believe she is ready, investigators will want to talk to her.
"It is out of the question to go and interview her in any sort of rushed way. She is extremely traumatised. Only the doctors have the ability to say (when she can be interviewed) and until I get the green light I will do nothing," Maillaud said.
It is hoped that Zainab, who had been enjoying a camping holiday with her family in the picturesque Haute-Savoie region of eastern France, could give a description of the attacker or attackers.
The possibility that she might be able to provide vital clues is regarded as all the more important now that police have given up hope of coaxing anything from her younger sister.
Zeena, who was physically unhurt in the attack, was only found eight hours afterwards, cowering beneath her mother's skirt in the family car where they were shot. A fourth victim -- a local man -- is believed to have been killed after stumbling across the scene by chance.
"All that time she (Zeena) was hiding, terrorised behind her mother's legs. She saw nothing," the prosecutor added.
In line with French law on questioning minors, the interview will have to be filmed.
Professionals involved in such work try to relax the child to encourage them to tell their story, in particular by using open ended questions.
"The idea is to lead the child to say things without asking too many questions. Often a child wants to please and there is a tendency to say 'yes'. This is tragic for an investigation," Maillaud told AFP.
Problems are compounded in the case of Zainab due to the need for a translater and possible memory loss she might have suffered due to her severe head injuries.
In the meantime, Maillaud said Zeena was "benefiting from all the psychological support available" in the hospital where she was being treated.
Relatives from Britain arrived in France on Saturday to see the two girls.
Maillaud, speaking to reporters in Annecy, said he could not give further details for security reasons but added that the children were as well as could be expected.
In the longer term, he said, it was hoped they would return to live with family members.
"Obviously at a certain time, the only legitimate destination would be in the arms of family."