Federal authorities are investigating repeated intrusions into the computer network that runs the Nasdaq stock exchange, according to a Wall Street Journal report that cited people familiar with the matter.

The intrusions did not compromise the tech-heavy exchange's trading platform, which executes investors' trades, but it was unknown which other sections of the network were accessed, according to the report.

"So far, [the perpetrators] appear to have just been looking around," one person involved in the Nasdaq matter told the Journal.

The Secret Service reportedly initiated an investigation involving New York-based Nasdaq OMX Group last year, and the Federal Bureau of Investigations has launched a probe as well. Investigators are considering a range of motives for the breach, including national security threat, personal financial gain, and theft of trade secrets, the newspaper reported.

Nasdaq representatives could not be reached for comment.

Investigators have not been able to follow the intruders' path to any specific individual or country, but people familiar with the matter say some evidence points to Russia, according to the report. However, they caution that hackers may just be using Russia as a conduit for their activities.

In 1999, a hacker group called "United Loan Gunmen" defaced Nasdaq's public Web site with a story headlined "United Loan Gunmen take control of Nasdaq stock market." The vandalism was quickly erased, and Nasdaq officials said at the time that the exchange's internal network was unaffected.