As the inquiry grows in complexity – the victims had direct links to four different countries – police in southeastern France have still been unable to trace a dark-coloured 4×4 vehicle spotted near the crime scene.

Officers have also searched woodland and remote hikers' huts for any trace of the killer.

An unnamed British man who alerted police to the shootings is said to have seen a car, and a motorcycle a few minutes later.

"That does not mean that this is necessarily the car involved," prosecutor Eric Maillaud cautioned however.

At first glance, the small parking area – about an hour on foot through forest along a narrow potholed road from the tiny village of Chevaline – would seem the perfect place to commit a crime.

But such a location would also have limited the possible escape routes.

The 'car park' is in fact an alcove-like space of about five by 12 metres (18×40 feet) edged on both sides by steep forested hillside.

Four bunches of flowers next to a pile of stones and some faint chalk marks on the road are now the only sign of the tragic events of last Wednesday.

Three members of a British-Iraqi family on a camping holiday on the shores of Lake Annecy and a local cyclist were found with two gun shots each in the head.

Saad al-Hilli, his wife Ikbal, from Surrey in southern England, and her mother all died.

The couple's two young daughters survived and police are still waiting to interview the elder, aged seven, who they hope will be able to describe what happened after she came out of a coma.

Just ahead of the car park is a road barrier – part of it now removed – which authorities say was not locked at the time of the killings.   

Beyond, a track leads off in one direction while to the left the road veers upwards in a hairpin bend.

Across the road, a stream runs along the bottom of a ravine with the terrain again climbing steeply on the other side.

For any assassin, an escape by road would have posed a high risk of being spotted by a walker or cyclist resulting in them being intercepted before reaching a main road.

The scene of the shooting is about 15 minutes' drive via Chevaline to a road leading to the resort town of Annecy and other major routes.

Indeed, the fourth victim, French cyclist Sylvain Mollier, is thought to have been killed after stumbling across the crime.

Although paved, the road from Chevaline is narrow and winding and it would not have been possible to drive at high speed, increasing the chance of a witness being able to give a detailed description.

In the other direction, paved road continues from the spot for about three kilometres before becoming a track for a further 2.5 kilometres (1.5 miles).

Another two kilometres of paved road then leads to the village of Jarsy and another main road.

From here, it is only around 60 kilometres (40 miles) to the Swiss border, at which there are no passport controls, and little more to Geneva and its international airport.

On foot, meanwhile, any escape would have been arduous and taken many hours.

Maps at the spot put the time needed to cover a distance of only 800 metres as the crow flies at six hours by forest path due to the circuitous route.

"Anyone who tried to escape through the forest would be stupid because they would have risked becoming encircled," Arnot Communal, a farmer who also runs some hikers' huts, told AFP by telephone.

"The police came here but we have not seen or heard anything except the police helicopter. That (an escape on foot) would surprise me very much," he added.

For now, investigators are pinning their hopes on seven-year-old survivor Zainab al-Hilli who has just emerged from a medically-induced coma to help her recover from a fractured skull.

Officers regard her as the key witness after younger sister Zeena, four, was unable to give any information. But it is not yet known how her injuries may have affected her ability to recall the horrific events of last week.