A family feud remains the main line of investigation in the murder of a British family and a cyclist in the French Alps one year on from the shootings, prosecutors said on Friday (September 6).

Three members of the Iraqi-born al-Hilli family were killed in a mountain lay-by not far from the town of Annecy in September last year in a quadruple murder that remains as yet unsolved.

British and French investigators meeting for a joint news conference in Annecy on Friday said a family disagreement centring on the father's older brother Zaid, 53, remained a key area of inquiry.

Saad, his wife and mother-in-law were shot in their car on September 5, 2012 -- along with a passing cyclist -- whilst their two young daughters escaped the killing although one sustained serious injuries.

Maillaud said that the two brothers had a historic disagreement regarding the mismanagement of their family estate, with Saad believing Zaid was taking advantage of his position for personal gain, prompting jealousy and resentment to build up.

The family estate included a house just outside London, two flats in the capital's suburbs and another in Spain, and a Swiss bank account along with other wealth in Baghdad, Iraq. In total the estate was valued at between three and five million euros.

But after twelve months of inquiry, investigators have still not reached firm conclusions, admitting much remains to be done.

Presented by Adam Justice