Three major US oil companies came under cyber attacks that may have come from hackers in China, the Christian Science Monitor newspaper reported Monday.

Sensitive bid data on oil discoveries by Marathon Oil, ExxonMobil, and ConocoPhillips were the target of the attack, the newspaper said citing documents and sources familiar with the investigation.

The attacks occurred in 2008, but their extent only became clear later after the FBI alerted the firms.

At least one attack was traced back to a computer in China, but it was unclear the extent of China’s involvement.

E-mail passwords, e-mails and other information were among the stolen data.

Internet giant Google earlier this month said it had been the victim of cyber attacks and said as a result it would review its operations in China.

China has rejected any involvement in such attacks.

“Accusations that the Chinese government participated in cyber attacks, either in an explicit or inexplicit way, is groundless and aims to denigrate China. We firmly opposed to that,” a spokesman of the Ministry of Industry and Information Technology was quoted as saying by the Xinhua news agency earlier Monday.