The chief prosecutor in the area of the French Alps where three Britons were gunned down is travelling to the UK as investigators continue to probe the mystery.

It comes as the victims' families released a statement in which they said they were "heartbroken" by the deaths.

Engineer Saad al-Hilli, 50, and his dentist wife Iqbal, 47, were brutally murdered along with her mother in a remote spot close to Lake Annecy a week ago. The couple's four-year-old daughter Zeena lay undiscovered under her mother's corpse for eight hours afterwards, while her seven-year-old sister Zainab was found with serious injures after being shot and beaten.

Annecy's chief prosecutor Eric Maillaud will arrive in the UK along with examining magistrate Michel Mollin, another senior member of the inquiry team.

They will join a small number of French investigators already in Britain to help unravel the mystery surrounding the deaths of the three Britons along with that of French cyclist Sylvain Mollier, 45, who apparently stumbled across the attack.

Mr Maillaud said on Wednesday that 40 French officers were working on the complex case, which has led to a flurry of theories relating to possible motives. But he gave no indication that French authorities were any closer to solving the murders, suggesting it could be years before answers emerge.

Investigators are focusing on three specific areas - Mr al-Hilli's work, his family and his native Iraq as they try to find a motive for the murders. Mr Maillaud said the large part of the investigation was taking place in the UK and that French authorities believe there could be a "great number of clues" in Britain. But he refused to be drawn on the main area of the inquiry's focus.

Mr Maillaud will visit the al-Hilli family home in Claygate, Surrey, as part of his 24-hour trip to the UK and will also meet with police officers and Crown Prosecution Service officials.

Wednesday's statement said the families had been "touched by the expressions of sympathy from people all over the world".

"We are very grateful for the support provided by the British, French and Iraqi authorities during this difficult time. We hope that those responsible for the deaths of our loved ones are brought swiftly to justice," it added.