Annecy chief prosecutor Eric Maillaud said there was nothing "that gives hope of a result in the near future" in the investigation into the deaths of Iraqi-born Briton Saad al-Hilli, 50, his wife Iqbal, 47, her mother Suhaila al-Allaf, 74, and a passing French cyclist, Sylvain Mollier, 45.
The al-Hillis' two children, Zainab, 7, and Zeena, 4, survived the shooting, though the elder sister was badly beaten. The youngest child was unscathed after hiding under the bodies of her mother and grandmother.
The family was shot at a car park while vacationing at a nature preserve in the Alps outside Annecy.
"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," Maillaud said. "There are lines of inquiry but each raises so many questions and nothing suggests there will be a quick solution."
French investigators have traveled to Britain to search al-Hilli's home, though it was unclear if they would travel to Iraq, The Daily Telegraph reported.
Investigators said Zainab told them only "one bad man" shot her family members. Police are looking into the possibility al-Hilli's connection to Iraq may be a factor.