The Federal Bureau of Investigation is probing a computer-security breach targeting Citigroup Inc. that resulted in a theft of tens of millions of dollars by computer hackers who appear linked to a Russian cyber gang, according to government officials.

The attack took aim at Citigroup's Citibank subsidiary, which includes its North American retail bank and other businesses. It couldn't be learned whether the thieves gained access to Citibank's systems directly or through third parties.

The attack underscores the blurring of lines between criminal and national-security threats in cyber space. Hackers also assaulted two other entities, at least one of them a U.S. government agency, said people familiar with the attack on Citibank.

The Citibank attack was detected over the summer, but investigators are looking into the possibility the attack may have occurred months or even a year earlier. The FBI and the National Security Agency, along with the Department of Homeland Security and Citigroup, swapped information to counter the attack, according to a person familiar with the case. Press offices of the federal agencies declined to comment.

Joe Petro, managing director of Citigroup's Security and Investigative services, said, "We had no breach of the system and there were no losses, no customer losses, no bank losses." He added later: "Any allegation that the FBI is working a case at Citigroup involving tens of millions of losses is just not true."

Citigroup is currently 27%-owned by the federal government.

The threat was initially detected by U.S. investigators who saw suspicious traffic coming from Internet addresses that had been used by the Russian Business Network, a Russian gang that has sold hacking tools and software for accessing U.S. government systems. The group went silent two years ago, but security experts say its alumni have re-emerged in smaller attack groups.

Security officials worry that, beyond stealing money, hackers could try to manipulate or destroy data, wreaking havoc on the banking system. When intruders get into one bank, officials say, they may be able to blaze a trail into others.

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