New York, Feb 5, (Reuters): Computer hackers repeatedly entered the network of the company that runs the Nasdaq stock market in the past year, but the trading platform was not compromised, the Wall Street Journal reported on Friday. 

The report cited people familiar with the matter as saying law enforcement investigators were considering a range of motives for the hacking, from financial gain to theft of trade secrets to a national security threat to damage the important stock exchange.

Authorities rank stock exchanges up with power companies and air traffic control centers as critical to the basic infrastructure of the United States.

The probe into the hacking at New York-based Nasdaq OMX Group Inc was started by the U.S. Secret Service and now includes the Federal Bureau of Investigation, the WSJ said.

A spokesman for Nasdaq OMX Group declined immediate comment on the report.

The WSJ said investigators have been unable to track the hacking to any specific individual or country.

"So far, (the perpetrators) appear to have just been looking around," the Wall Street Journal quoted one unidentified person involved in the Nasdaq case.

It reported that another person familiar with the case said the incidents were, for a computer network, the equivalent of someone sneaking into a house and walking around but so far not taking anything or tampering with anything.