The European Commission's antitrust authorities carried out unannounced inspections at offices of at least three oil companies in three countries on Tuesday as part of a probe into suspicions of price-fixing.

BP PLC (BP), Royal Dutch Shell PLC (RDSB.LN) and pricing agency Platts all confirmed that their offices had been raided. The raids also included Norway's Statoil ASA (STL.OS). Norway is not a member of the EU but abides by its competition rules as part of its membership in the larger European Economic Area.

"The Commission has concerns that the companies may have colluded in reporting distorted prices to a price reporting agency to manipulate the published prices for a number of oil and biofuel products," the Commission said in a press release.

Competition authorities are also investigating if the oil giants prevented other companies from participating in the price assessment process in efforts to distort published prices. All of the companies announced that they are cooperating with the investigation.

"Several companies may have participated in anti-competitive agreements," Statoil spokesman Jannik Lindbaek Jr. told Dow Jones Newswires. "In addition, the inspection is about the potential abuse of a dominant position by another company."

Commission officials are investigating prices provided to Platts. The world's leading oil pricing agency's parent company, The McGraw-Hill Companies, Inc. produces information and benchmark price assessments for energy markets. Platts' physical market assessments form the basis of billions of dollars worth of oil sales around the world.

Mr. Lindbaek said Statoil has been providing Platts with daily price data for crude oil, refined products and biofuels since 2002. However, terms of such agreements aren't required to be made public, unlike the market for oil futures, which are visible on public exchanges. Platts is one of the main sources of prices and it relies on traders to give them details of deals in the market.

"Even small distortions of assessed prices may have a huge impact on the prices of crude oil, refined oil products and biofuels purchases and sales, potentially harming final consumers," the commission said.

The EC said that unannounced inspections are a preliminary step to investigate suspected anticompetitive practices. "The fact that the Commission carries out such inspections does not mean that the companies are guilty of anti-competitive behaviour nor does it prejudge the outcome of the investigation itself."