The European Commission said in a statement that even small distortions of assessed prices could have a "huge impact on the prices of crude oil, refined oil products and biofuels purchases and sales, potentially harming final consumers".
The statement said:
The European Commission can confirm that, on 14 May 2013, Commission officials carried out unannounced inspections at the premises of several companies active in and providing services to the crude oil, refined oil products and biofuels sectors.
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.
Furthermore, the Commission has concerns that the companies may have prevented others from participating in the price assessment process, with a view to distorting published prices.
Any such behaviour, if established, may amount to violations of European antitrust rules that prohibit cartels and restrictive business practices and abuses of a dominant market position (Articles 101 and 102 of the Treaty on the Functioning of the EU and Articles 53 and 54 of the EEA Agreement).
More top news
The England cricket captain is the first Englishman and youngest player to ever achieve the landmark.
Police are appealing for news of Connor Jarvis, 19, after he did not return from a solo holiday in Marrakech as planned.
The Leicester City ace said he "felt himself starting to go" when he saw his bride on their magical wedding day last week.