Citibank has been fined $4.5 million by the Commodity Future Trading Commission (CFTC) for a design flaw which led to the deletion of millions of audio files, including recordings which had been subpoenaed.

The CFTC said it had received an assurance from Citi that audio files requested for an ongoing investigation into a number of traders would be preserved.

However, when the regulatory body came calling for the files following a two month deferment, Citi reported that the recordings, totalling more than 2.77 million audio files for 982 users, had been scrubbed three weeks earlier.

The bug in the system had been notified to senior staff in a staff memo in 2014, which warned that if it was not configured correctly, there was a “ticking time bomb effect” that could, and in this case did, lead to the automatic deletion of audio recordings.

“Registrants have obligations to diligently supervise all aspects of their business related to their duties, including all systems used to comply with CFTC recordkeeping requirements, document requests, and subpoenas,” says division of enforcement director James McDonald. “These regulations exist to promote the integrity of the marketplace. When registrants fail to meet their supervision obligations, they will be penalized.”