A single semi-automatic weapon was used to shoot dead four people in the Alps last week, including the parents and grandmother of two British girls who survived the attack, French state prosecutor Eric Maillaud said on Monday.
Police searching the family's home in Surrey, to the south of London, briefly widened a security cordon around the site and called in bomb disposal experts but found nothing hazardous.
"Yes, it's confirmed," Maillaud told Reuters when asked about reports of a 7.65 millimeter automatic pistol being the only weapon used in the multiple murder on a remote mountain road near the border with Switzerland and Italy.
Saad al-Hilli, the Iraqi-born British driver was shot twice in the head in his car along with his wife, her mother and a passing cyclist last Wednesday on the road near the village of Chevaline, not far from the picturesque Annecy lake where the Hilli family were staying at a campsite on holiday.
About 25 gun shells were retrieved from the area and the corpses of the four victims suggesting that there could have been more than one gunman.
Maillaud has repeatedly declined to say whether police were searching for one assailant or more.
Earlier on Monday, officers sealed off the road in the leafy village of Claygate in Surrey, southwest of London, and began removing neighbors living close to Hilli's 1 million pound ($1.60 million) house after finding suspicious substances in the back garden.
"A bomb disposal unit was called to the scene to carry out an assessment as a precautionary measure." a Surrey police spokeswoman said, adding that the search of the property was continuing. "Surrey Police can confirm that items found at an address this morning in Oaken Lane, Claygate are not hazardous."
Monday's search was the latest development in a shooting that has dominated media headlines in Britain and generated much speculation as to the motive.
Hilli, a mechanical engineer who worked with Surrey Satellite Technology, a subsidiary of aerospace and defense firm EADS, and the other victims were shot in what appeared to be execution-style killings.
His older daughter Zainab, 7, who suffered serious skull fractures, came out of a medically induced coma on Sunday and will be questioned as to what happened as soon as she is able, Maillaud has said.
Her younger sister Zeena, 4, survived the shooting unscathed and returned to Britain on Sunday.
Investigators have said they were looking at various theories, including robbery, a family feud, a possible link to Hilli's work in the aerospace industry or his Iraqi origins.
(Additional reporting and writing by John Irish in Paris; Editing by Louise Ireland and Brian Love)