Statoil has confirmed a raid took place at its headquarters in Stavanger, Norway, earlier today.
Statoil said the inspection was carried out by the EFTA Surveillance Authority with the assistance of the Norwegian Competition Authority.
A statement on the firm's website said the inspection was carried out "on request of the European Commission".
"Statoil is cooperating with the authorities in their inspection", it added.
BP said it is "cooperating fully" with the EU's anti-trust body after it launched an investigation into allegations of oil price fixing.
A spokesperson said: "BP is one of the companies that is subject to an investigation that was announced earlier today by the European Commission.
"We are cooperating fully with the investigation and unable to comment further at this time", they added.
Shell has confirmed it is "fully cooperating" with the European Commission after the EU's anti-trust body launched an investigation into claims of oil price fixing.
A spokesperson said: “We can confirm that Shell companies are currently assisting the European Commission in an enquiry into trading activities.
"We are fully cooperating with the investigation. For legal reasons we cannot make any further comment at this stage”, the spokesperson added.
Oil giant Shell has announced bumper profits today of just under eight billion US dollars (£5.1 billion) for the first quarter of 2013.
It represents an increase of 4% from one year ago, while share prices also rose in the new figures.
The Anglo-Dutch company has weathered volatility in energy markets, including a recent drop in oil prices, with improved profits from refining and trading.
The oil and gas industry will play a vital role in British energy needs for decades to come, according to three Liberal Democrat ministers.
Business Secretary Vince Cable, Scottish Secretary Michael Moore and Energy Secretary Ed Davey were in Aberdeen this morning to set out their long term strategy, aiming to boost investment and supply chains while promoting exports.
Mr Davey said the industry will play a huge role despite efforts to promote cleaner fuels and decarbonisation.
"The primary products, oil and gas, on all estimations will remain an integral part of the mix for many decades to come," he told an invited audience which included industry leaders and local politicians.
"Only the UK can deliver what is required over a sustained period if you are going to get the most out of the oil and gas industry. The UK is a large economy - that is why we can provide the support.
"Smaller economies have difficulty absorbing the costs.
"The size of the UK economy can really create the framework and certainty."
An "extreme" engineering centre will be built to help develop deep sea technology.
The £7 million Neptune National Centre will be created on the River Tyne in Newcastle.
Unveiling the centre as as part of a new oil and gas strategy, Business Secretary Vince Cable said: "This cutting-edge new facility will help put our academic community and industry at the centre of subsea and offshore engineering research."
Newcastle University will help provide funds of try scheme as a centrepiece for economic revival.
Energy Secretary Ed Davey said the oil and gas industry is a vital strategic resource.
"With our support for carbon capture and storage, for decommissioning and by encouraging increased collaboration across different energy sectors, especially offshore, there will also be new sustainable growth opportunities for the industry and the wider UK supply chain."
Scottish Energy Minister Fergus Ewing said he welcomed the UK Government view that there is a long-term future for the industry.
He said that the UK strategy highlighted oil and gas as an "enormous economic resource for decades to come.".
At an announcement on the UK's oil and gas strategy in Aberdeen today, Business Secretary Vince Cable is expected to say:
"I want us to consider what barriers are stopping British companies bidding for and winning work in the North Sea.
"This is an expanding industry.
"We can either help create more jobs and opportunities across the UK if we get this right, or see work going overseas if not."
The Government will today announce an oil and gas strategy that aims to secure thousands of jobs and billions of pounds of investment.
Plans for "tax certainty", skills development and supply chain support will be discused by three secretaries of state in Aberdeen, the oil capital of Europe.
Business Secretary Vince Cable, Scottish Secretary Michael Moore and Energy Secretary Ed Davey will speak to business leaders about the strategy this morning.