Annecy prosecutor Eric Maillaud denied media reports suggesting the DNA could be that of the still unidentified killer, saying it belonged to a ballistic expert who handled material that had already been thoroughly analysed by police.
"No other DNA trace has been found at the moment," he said.
The contamination has, however, prompted police to collect elimination samples from investigators, rescue workers, medical staff and laboratory technicians in case more elements are contaminated.
Police in France have been struggling to solve the case of the shootings last year in the Haute-Savoie department, where Saad al-Hilli, his wife Iqbal and her mother Suhaila al-Allaf were all found dead inside their estate car near Lake Annecy on September 5, along with a French cyclist who investigators believe was an innocent bystander.
The couple's two young daughters survived the attack, which took place in the village of Chevaline, about 150 kilometres (90 miles) from Daillon.