Police in France have found two mobile phones inside the car in which three people, including a British couple, were shot dead.
Sky's crime correspondent Martin Brunt, who is in Annecy where the murders took place, said the phones could explain how and why Saad al Hilli and his wife Iqbal died, alongside a Swedish woman and a French cyclist.
"Did they make any calls in the preceding hours or days that might have involved a rendezvous in that remote layby halfway up the mountain to meet someone they knew?" he asked.
Brunt said the phones would also tell the police if calls had been made to the emergency services as the attack happened.
The couple's two daughters survived the attack and are being treated in hospital in France.
French and British police will today conduct further searches of the al Hilli family home in Claygate, Surrey, as they look for a motive for Wednesday's murders.
Officers began a detailed search of the family's mock Tudor property yesterday.
They placed a tent in the driveway of the house and also took evidence-gathering material, including boxes and bags, into the property as well as photographing the exterior.
One forensics officer took various pieces of equipment into the property, including an angle grinder which could be used to access locked cupboards or a safe.
During a news conference in France, prosecutor Eric Maillaud said each of the four people who died was shot twice in the head, and he hoped the authorities would "solve this awful drama as quickly as possible".
French detectives from the Haute-Savoie region widened their investigation by travelling to the UK.
In a joint statement by British and French officers outside Woking police station, Colonel Marc de Tarle said the investigation was "long and complex" but that the cooperation between both countries was going smoothly.
In Surrey, a technician from a local security firm was called to disable the burglar alarm that sounded shortly after detectives went into the detached house.
Former Metropolitan Police detective Peter Bleksley told Sky News: "It was a visit more than a search for the French officers.
"They've left it in the hands of the forensics experts who are in the blue and white overalls and will pick through this house.
"Our homes tell the stories of our lives and there could be all sorts of things that might offer up unknown facts or family secrets."
French investigators also plan to interview Mr al Hilli's brother - who has approached UK police to deny any feud between the siblings over money.
Mr Maillaud said the police would be speaking to all immediate family members about the killings.
Iraqi-born Mr al Hilli, 50, was shot dead in his BMW alongside his dentist wife while on holiday close to Lake Annecy.
An older Swedish woman who was travelling in the car was also killed, along with Sylvain Mollier, 45, a French cyclist who apparently stumbled across the attack in Chevaline, near the borders of Italy and Switzerland.
Around 40 French investigators are working around the clock on the case. Swiss and Italian police are also helping in the hunt for those behind the shootings.
The couple's four-year-old daughter Zeena lay undiscovered under her mother's corpse for eight hours after the murders, while her seven-year-old sister Zainab remains in a medically-induced coma after being shot and beaten.
There has been speculation the Swedish woman is the children's grandmother but this has not been confirmed.
Two close family members of the young girls have arrived in the region with a British social worker. They will be allowed to visit the children, but only under the supervision of detectives.
The two orphans have been in the care of British consular staff and nurses.
Mr Maillaud said Zeena has been looked after by psychiatric teams and had spoken about what he described as the "terror" of what happened, but did not see anything because she was hiding.
Zainab is not yet well enough to be interviewed, but it is hoped she will be able to provide vital details of the attackers.