A Citigroup (NYSE:C) staffer's fat-finger trade could cost the investment bank losses of $50 million, Bloomberg reported Thursday afternoon.

Last month, a Citigroup employee caused a flash crash in European stocks following a mistaken trade.

According to Bloomberg, citing people familiar with the matter, Citigroup is still calculating the losses, and it could rise above $50 million.

"A trader in the firm’s Delta One trading unit in London was working from home during a bank holiday on May 2 when the person incorrectly added an extra zero to a trade early in European market hours," Bloomberg said, quoting a source.

The blunder resulted in a significant sell-off across European equities, erasing 300 billion euros ($322 billion) at one point.

The employee who made the error is said to have been placed on leave, with recent reports stating Citigroup is looking to overhaul their London trading team linked to the error.