According to reports, investigators were seen entering Lactalis' offices in Laval, western France Wednesday morning.
Magistrates and 70 police were searching company headquarters as well as a factory in nearby Craon identified as the source of the tainted milk.
Pressure rose on Lactalis Monday as the parents of babies who became sick after drinking salmonella-laced milk demanded answers over a scare affecting dozens of countries.
Lactalis CEO Emmanuel Besnier said at the weekend that more than 12 million packages of Picot, Milumel, Celia and other brands of powdered baby milk had been recalled in 83 countries and offered to compensate all the affected families.
An association of victim's families, which met with the government on Monday, has rejected the offer, accusing Lactalis of trying to buy their silence.
The association's president Quentin Guillemain said Monday the explanations given by Besnier in an interview Sunday -- his first since the outbreak in December fell far short of expectations.
"We still don't know where they are, we don't know if they have been destroyed or if they've been drunk," he said.
He said it also remained unclear when the salmonella outbreak at Lactalis's Craon plant in western France first occurred, suggesting it could have been before 2017, the period initially covered by the recall announced in December.
"It's a question we asked once again, and as of now we have not had any response," said Guillemain, who has demanded an apology from Lactalis.
His group has disputed health authorities' tally of 37 children sickened by the salmonella outbreak in France, saying that without systematic testing of babies brought to doctors, the true figure remains unknown.
Hundreds of families have filed lawsuits against the company.