The European Commission has carried out surprise inspections at several major oil companies over possible price manipulation in breach of EU anti-trust rules.
The Commission did not name any of the companies involved in Tuesday's raids, but British energy giant BP, Shell and Norway's Statoil said that their offices had been visited.
"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 EC said.
Statoil said a raid at its headquarters in Stavanger, Norway, was carried out with the assistance of Norwegian anti-trust officials.
BP and Shell offered no details, but said they were cooperating with authorities.
"We can confirm that Shell companies are currently assisting the European Commission in an inquiry into trading activities," a Shell spokesman said.
Platts, the world's leading oil price reporting agency, also confirmed it was being investigated.
The raids represent one of the biggest cross-border actions since the Libor rigging scandal.
The inspections took place in two European Union member states and one non-EU country, the EC said.
"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," it said.
The EC also said companies may have prevented others from participating in the price assessment process, with a view to distorting published prices.
Statoil said the suspected violations were related to the Platts price assessment process and may have been ongoing since 2002.
The probe will shine a light on the methodology designed by Platts for daily assessments on the physical oil markets, used to close deals worth billions of dollars.
The Commission said that 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.
It added the fact inspections had been carried out did not mean the companies were guilty of anti-competitive behaviour.
The EC did not list the companies being investigated, and it was not clear whether other companies were included.