The European Union's executive arm, the Commission, on Tuesday said it had raided the offices of a number of oil industry companies for possible price-fixing.

Here are some questions and answers on the investigation.

_ Which companies were raided and why?

The EU Commission did not say which companies it is investigating. However, some firms have confirmed they are part of the probe. They include Britain's BP, Royal Dutch Shell, which is listed in London and Amsterdam, and Norway's Statoil. Platts, a division of McGraw Hill Financial that compiles and provides data and news for the energy market, also said its offices in London were raided.

The three oil companies are all major producers in the international energy industry and contribute data to Platts' Market on Close (MOC) pricing process, which every day publishes the final trading price for numerous commodities.

The EU said it has concerns that some companies may have tried to manipulate the pricing process by colluding to report distorted prices and by preventing other companies from submitting their own prices.

_What prices are suspected of being rigged?

The Commission has not specified exactly which contracts are being investigated. It said, however, that its probe covers a wide range of oil products _ crude oil, biofuels, and refined oil products, which include gasoline, heating oil, petrochemicals and others.

_What role do price reporting agencies like Platts play in setting the market prices?

Every day, Platts compiles the bid, offer and transaction prices for trades in oil products. The information is voluntarily provided by oil companies, traders, large consumers like airlines or utilities, and other market participants.

Unlike oil futures, which set prices for contracts, the data used in the MOC process is based on the physical sale and purchase of actual shipments of oil and oil products.

After gathering the information, Platts editors analyze the data and estimate a price for each oil product at the end of each trading day. That information is used by other companies as a benchmark when they sell their products _ such as gasoline or jet fuel _ to customers in the open market.

There are other companies, such as Argus Media and ICIS, which do similar work setting commodity prices.

_How and how long do the authorities think the prices have been rigged?

According to Statoil, the EU investigation stretches back to 2002, which is when Platts launched its MOC price system in Europe. The suspicion is that some companies may have provided inaccurate information to Platts to affect the oil products' pricing, presumably for financial gain.

_Does this mean gas prices at the pump were inflated?

The EU Commission says that is possible. In announcing its investigation, it said that "even small distortions of assessed prices may have a huge impact" on the price of commodities like gasoline, which "could potentially harm final consumers."

At the same time, oil prices are affected by multiple factors that affect traders' willingness to buy oil products. They include indicators on the state of the global economy; conflict in the Mideast, where a lot of crude is produced; the strength of the dollar, which is used to trade oil products; and hurricanes affecting rigs and refineries near the Gulf of Mexico.

So the extent to which the price of an oil product _ such as gas for your car _ can be manipulated through the MOC process is relatively small.

_What happens next?

The EU Commission says there is no legal deadline for finishing the investigation and it could take years before its final conclusions are known.

In the meantime, however, pressure is likely to grow for tougher regulations overseeing each participant in the pricing process, from the price reporting agencies to traders.

According to a report from analysts at IHS Global Insight in London, the fear of negative publicity could also make companies decide to refrain from submitting information to the pricing process, leading to even less transparency and weakening the effectiveness of the pricing agencies.