Three U.S. oil companies were targeted in 2008 in computer attacks in which sensitive information was leaked, including in one case to a computer in China, according to a published report.

In the attacks, senior level executives received e-mails that contained embedded links that when clicked on downloaded spyware to computers, The Christian Science Monitor reported Monday. The spyware was custom-made and undetectable by antivirus software, according to the report. The publication conducted a five-month investigation into the attacks.

The companies--Marathon Oil, ExxonMobil, and ConocoPhillips--were informed by the FBI that valuable information was targeted including "bid data" on the quantity, value, and location of oil discoveries around the world, and messages and e-mail passwords were exposed, the report said.