Citigroup Inc. elaborated on its denial that its systems had been breached last summer, suggesting that, if a breach occurred, it would have happened at a third party.

"As with virtually all financial institutions, there are instances of fraud or breaches of third-party systems that result in our taking actions to protect our customers and Citi … , [but] there has been no breach of Citi's systems," the New York company said in a press release issued Dec. 21.

It did not identify any third party that might have had a breach.

The Wall Street Journal reported that morning that the Federal Bureau of Investigation has been looking into a Russian hacker group that targeted Citi during the summer. Citi was quoted in the story as denying it.

The paper cited unnamed government sources who said the hacker group stole tens of millions of dollars from its target.