A photographer suspected of taking topless vacation pictures of Catherine, the Duchess of Cambridge, last year was placed under formal investigation in June, a spokeswoman for the Nanterre prosecutor said Friday.
A second photographer is also expected to be placed under formal investigation "soon," the spokeswoman said.
The editor of the French edition of Closer magazine, Laurence Pieau, was placed under formal investigation in July for having published the photographs last September, according to the prosecutor's spokeswoman.
The decision by the French Closer magazine to publish the photographs of Catherine, taken while she was on vacation with Prince William last September, sparked a debate over privacy -- and legal action by the royals.
Mondadori France, the magazine's publisher, was placed under investigation earlier this year, as were an unnamed female photographer and La Provence, a regional daily newspaper which also published the photographs.
The grainy topless pictures, shot from a distance, were snapped while Catherine was sunbathing in private during a vacation at a chateau owned by William's uncle in Provence, southern France.
Closer was fined last September for publishing the topless photographs and ordered not to distribute the magazine in print or online. A French court also ordered the magazine to hand over the original photos to the royal family within 24 hours of the ruling.
The photographs subsequently appeared in outlets in other countries, including Denmark, Italy and Sweden.
No media outlet in Britain published the images.
The Duchess of Cambridge made headlines for a different reason this week, having given birth on Monday to a son, Prince George. The royal baby, whose arrival was greeted with great fanfare by the media, is third-in-line to the throne.