There will be no quick solution to finding the killers of four people gunned down in the French Alps, a prosecutor warned.

Around 100 police officers in Britain and France are investigating the deaths of engineer Saad al Hilli, 50, his dentist wife Iqbal, 47, her elderly mother and passing cyclist Sylvain Mollier in the attack last month.

The couple's four-year-old daughter Zeena lay undiscovered under her mother's corpse for eight hours after the shooting, while her seven-year-old sister Zainab was found with serious injures after being shot and beaten.

They were attacked as they drove through a remote area close to Lake Annecy and so far there have been no clear leads.

Annecy's chief prosecutor, Eric Maillaud, said: "It's a long task and there's nothing that gives hope of a result in the near future."

French investigators came to the UK and searched the al Hilli family home in Claygate, Surrey, in the wake of the deaths.

They said last month they were investigating three lines of inquiry, focusing on Mr al Hilli's work, his family and links to his native Iraq.

Mr Maillaud said: "Perhaps if we can understand why they were killed we can work out who killed them but at the moment there are many questions.

"I think the investigation will take a very, very long time, unless we discover something that will suddenly enable us to understand everything.

"There are lines of inquiry but each raises so many questions and nothing suggests there will be a quick solution."