Lactalis' spokesman Michel Nalet said the group was ready to collaborate with judicial authorities.

"We will provide all necessary elements so that the investigation can run smoothly," he said.

France's Lactalis, one of the world's largest dairy groups, has been at the centre of a health scare over the past few weeks after three dozen babies fell ill in France as a result of contaminated baby milk produced by the company.

The Lactalis crisis deepened when it emerged some of the recalled baby foods had still made their way on to shop shelves in a number of supermarket chains.

The Paris prosecutor opened a preliminary probe into the salmonella contamination scare in late December.

Police searched the group's headquarters in Laval, southwest of Paris, the Craon factory where the tainted baby milk was produced, and the offices of Lactalis Nutrition Sante and Lactalis Nutrition Dietetique in Torce, Brittany, which hosts the group's quality controls services, the source said.

They also went to an annex of the company's headquarters located in Change, near Laval and the headquarters of Lactalis International Europe near Paris.

"These are technical searches for data and documents, to understand the manufacturing process and the flaw that allowed the contamination," another source close to the investigation told Reuters.

(Reporting by Emmanuel Jarry, Writing by Sarah White and Sybille de La Hamaide, Editing by Richard Lough and Jane Merriman)