A Czech-made machine gun is thought by gun experts to be the one used in an attack that orphaned two British children.
Police are satisfied that just one weapon was used in the attack close to Lake Annecy – reportedly a 7.65-calibre gun.
Experts said the nature of the attack on the al Hilli family, who lived in Surrey, and a cyclist who stumbled upon the scene “pointed strongly” to the Skorpion.
The weapon was developed in the 1950s for use by security and special forces and is reportedly still used as a weapon by armed forces in some countries.
All four who died were shot twice in the head.
“The Skorpion is easily reloadable, doesn’t sound very loud, which might explain why nobody as far as we know heard the attack on the al Hilli family.
“It’s a gun which holds 20 rounds and we know that there were 25 bullet casings found at the scene…
“All this fuels the idea that this was an attacker who knew exactly what he was doing.
“And if not a professional, it was someone who was operating in a very clinical and methodical way.”
French officials said that despite media reports of the calibre of the weapon, they denied having confirmed “the nature, calibre and number of weapons used”.
Meanwhile, French officers are now combing through CCTV as they attempt to trace a dark 4×4 vehicle spotted near the scene.
They have also been searching woodland and remote hikers’ huts for any trace of the attacker, who is believed to have been acting alone when he struck at a secluded car park in the Combe d’Ire forest, near Chevaline, fuelling speculation they were targeted by a contract killer.
Saad al Hilli, 50, from Surrey, was killed in the family car alongside his dentist wife Iqbal during their holiday.
Her mother also died in the shooting along with Sylvain Mollier, 45, a French cyclist who apparently stumbled across the shooting on Wednesday.
The couple’s seven-year-old daughter, Zainab, was shot and so brutally beaten during the attack that doctors placed her in a medically-induced coma.
She has since regained consciousness and was able to hold a brief discussion with officers in France, sources close to the investigation have said.
Zainab is seen as a key witness to the horrific attack, along with her younger sister, four-year-old Zeena, who was not shot and has now been flown back to Britain with carers.
It is unclear who will take custody of the two orphaned children.
French police are also examining two mobile phones found in the bullet-ridden BMW, just a few miles from Le Solitaire du Lac, a campsite in Saint-Jorioz where they were staying.
Witnesses have also described seeing a motorbike in the area.
An investigation at the al Hilli family home in the affluent village of Claygate, Surrey, appears to be winding down after British police identified items of concern and a bomb disposal squad was called in.
Neighbouring properties were evacuated as police focused on a shed at the bottom of the garden. Officers later said that the unidentified items were not hazardous.
Police were later using power tools to try to open a safe in the al Hilli home.
Some media reports have suggested that Mr al Hilli, an engineer who left Saddam Hussein’s Iraq years ago, was known to the security services and was put under surveillance by Metropolitan Police Special Branch during the second Gulf war.
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