European Union antitrust authorities Tuesday carried out unannounced inspections of giant energy companies BP PLC, Royal Dutch Shell PLC and Statoil AS A, escalating a probe of alleged oil-price manipulation amid scrutiny of how prices are set in global financial markets.

EU officials are investigating whether the three companies manipulated prices in the $2.5 trillion physical-oil market by giving false data to an oil index publisher, the Platts unit of McGraw Hill Financial Inc. Platts's offices were also inspected Tuesday.

Index-publishing firms like Platts derive their prices from self-reported transaction data from participants in deals.

It isn't clear whether Platts is a target of the probe, nor whether the investigation is limited to those four companies. Representatives of the oil companies and Platts said they are cooperating with authorities.

The investigation comes months after a handful of the world's largest banks paid more than $2 billion to settle allegations they falsely reported borrowing costs to influence the London interbank offered rate, or Libor, and boost their trading profits.