NEW YORK: A US regulator on Monday fined Citigroup Inc $4.5 million for deleting millions of audio files, including recordings it had subpoenaed, after failing to fix a known design flaw in its audio preservation system.

The US Commodity Futures Trading Commission said the Nov. 2018 deletion of 2.77 million audio files included recordings of traders subpoenaed in Dec. 2017 for a probe, which Citigroup had promised would be preserved.

Citigroup was accused of not following up on an employee's warning in 2014 that the configuration of its audio preservation system might create a "ticking time bomb effect" that could, and did, cause mass deletions of audio files.

The civil fine was imposed against Citibank NA, Citigroup Energy Inc and Citigroup Global Markets Inc for the New York-based bank's failure to maintain sufficient internal controls over its technology.

Citigroup did not admit or deny wrongdoing. A spokeswoman said the bank was pleased to settle.

According to the CFTC, the deleted recordings comprised 683,296 calls for 982 individual users, including recordings covered by its subpoena, and 2,087,789 recordings from speakerbox or "Hoot n' Holler" lines.

The case is the latest to highlight problems with technology systems at the third-largest US bank by assets.

Citigroup is battling separately in Manhattan federal court to recoup $900 million of its own money that it accidentally paid to lenders for the cosmetics company Revlon Inc.

Some hedge funds that lent money to Revlon and received payments from Citigroup have balked at repaying the bank, which blamed the incident on a clerical error.