It could still take months before a decision is made on whether to charge Mondadori Magazines France and local photographer Valerie Suau with the criminal exploitation of the images. Suau's employer, local newspaper La Provence, was also placed under formal criminal investigation.
The photos, which appeared in the French Closer magazine last September, showed the Duchess of Cambridge relaxing at a private villa in Provence, in southern France, sometimes without her bikini top and, in one case, her suit bottom partially pulled down to apply sunscreen.
If convicted, Closer magazine could face closure for up to five years; the photographer could face fines and a year in prison. The potential punishment for La Provence was not yet clear.
Meanwhile, with her baby bump more visible than ever, Kate visited the National Portrait Gallery on Wednesday and spoke to a group of children about the power of art to make a difference.
The Associated Press contributed to this report.