The European Commission has confirmed it raided several companies involved in ebook publishing.
The regulators said it carried out "unannounced inspections" at ebook publishing companies in several member states because it has reason to believe they may have "violated EU antitrust rules that prohibit cartels and other restrictive practices".
European officials were accompanied by local competition regulators. The UK's Office of Fair Trading started investigating ebook pricing last month.
The Commission said the raids were just a first step and not necessarily evidence of guilt.
It said there was no deadline for the investigation. The length of the probe depends on complexity of the case and the cooperation of all the parties involved.
Many observers expected the arrival in Europe of Amazon's Kindle, and therefore increasing electronic sales, to result in falling prices for ebooks.
But the big publishing houses successfully stood up to Amazon last year to force an "agency-pricing" model on electronic distribution of books.
This model is broadly similar to the Net Book Agreement; publishers set prices for a set number of books. The NBA was killed off by the Office of Fair Trading. ®