Russian hackers had infiltrated Dow Jones & Co to steal information to trade on before it was made public, and the breach was "far more serious than a lower-grade intrusion" disclosed by the company, Bloomberg reported, citing sources.

The Federal Bureau of Investigation, Secret Service and the Securities and Exchange Commission are leading an investigation, which began at least a year ago, Bloomberg reported.

"We have received no information from the authorities about any such alleged matter," Dow Jones spokeswoman Colleen Schwartz said in an email, adding that the company was looking into the report.

Dow Jones, the publisher of the Wall Street Journal and a unit of Rupert Murdoch's News Corp, disclosed last week a breach of its systems that put payment card and contact information of about 3,500 individuals at risk.

Dow Jones had said that there was unauthorized access to its systems at certain times between August 2012 and July 2015.

"We are aware of the Dow Jones intrusion and looking into it," FBI spokeswoman Kelly Langmesser said via email.

The hackers sought information including stories being prepared for publication, Bloomberg said on Friday, citing two people familiar with the investigation.

The Secret Service could not be immediately reached for comment on the Bloomberg report. The SEC declined to comment.

(Reporting by Supantha Mukherjee, Jessica Toonkel and Joseph Menn; Editing by Savio D'Souza and Sriraj Kalluvila)