According to the report, which cites anonymous sources, Nasdaq OMX Group computers were compromised sometime over the past year, but the company's trading platform was unaffected. "So far, [the perpetrators] appear to have just been looking around," the Journal quotes one source as saying.

Nasdaq OMX Group runs a number of stock exchanges, including the U.S. Nasdaq, and exchanges that trade in Copenhagen, Stockholm, Helsinki, and the Baltic region. The investigation is being conducted by the U.S. Federal Bureau of Investigation and the U.S. Secret Service, the report states.

Nasdaq representatives could not be reached immediately for comment.

Hacking incidents like this are becoming increasingly common, as widespread e-mail and Web-based computer attacks continue to overwhelm workers in corporations and in government. In some cases, hacking into a low-level employee's desktop can be the first step for a more sophisticated attack, but often the cybercriminals go no further into the network.

The report is worrying, however, as it comes just days after the London Times reported that police are investigating whether may have been connected to a cyber crime. Authorities are also looking into that moved stock prices wildly for five companies, including BT and Next, the Times reported.