There will be no "quick solution" in finding the killers of four people who were gunned down in the French Alps, a prosecutor said.
About 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 horrifying 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 as to why the four were massacred.
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 that 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: "We're investigating everything but it all takes a lot of time, trying to piece together the lives of all the people who have died, trying to perhaps understand a real motive, the real reasons for these killings.
"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."
He said it was too early to say whether officers would travel to Iraq as part of the inquiry.