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
Consumers have been warned to immediately stop using Indesit tumble dryers after a machine reportedly caused a huge fire in a tower block.
Some 29 children have been killed in Aleppo in the week leading up to US-Russia talks which aid workers hope will lead to a vital ceasefire.
The 23-year-old was greeted by hundreds of fans as has returned to his gym in Essex for the first time since victory in Rio.