The US Federal Bureau of Investigation is investigating a hacking that targeted Citigroup and resulted in the theft of tens of million of dollars, according to the Wall Street Journal. Its report quoted US government officials.

The cyber attack by hackers believed to be linked to a Russian cyber gang was aimed at Citi's Citibank subsidiary, the paper said, adding it was unclear whether the hackers gained access to the bank's systems directly or through third parties.

Citigroup did not immediately respond to this publication’s enquiries on the matter. The FBI could not be reached for comment.

The report did not specify whether any other parts of Citigroup, such as its private bank, were affected.

Two other entities, including a US government agency, were also attacked by hackers, the paper said, citing people familiar with the attack on Citibank.

The cyber attack on Citibank was believed to have taken place last summer. However, the report said the attack could have happened 12 months’ earlier.