Two Citigroup traders who made millions of profits by spoofing in the U.S. Treasury futures markets have been fined around $550,000 by the U.S. Commodity Futures Trading Commission (CFTC) and also agreed to serve a 6-month suspension from trading. The abuses of Citi traders allegedly occurred between July 2011 and December 2012.

Stephen Gola and Jonathan Brims allegedly manipulated the U.S. Treasury futures market more than 1000 times through placing and quickly cancelling bids to manipulate the prices, an illegal process known as ‘layering’ and ‘spoofing’. The 2010 Dodd-Frank Act, which made the practice illegal, defines it as “bidding or offering with the intent to cancel the bid or offer before execution.”

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Regulators and exchanges have stepped up their policing of spoofing in recent years, however the people and firms they previously focused on were rather small-time avid gamers in markets. Earlier in January, regulators also ordered Citigroup to $25 million fine to settle charges it spoofed the Treasury futures market, the biggest spoofing settlement to date.

In conjunction with the $25 million civil monetary penalty, Citigroup was also required to cease and desist from violating the Commodity Exchange Act’s prohibition against spoofing and the CFTC regulation governing diligent supervision.

The aforementioned individuals’ spoofing strategy typically meant placing a small order which they intended to trade, while also making a series of large orders, bids or offers of 1,000 lots or more, which they had no intention of completing.

Spoofing in general is a practice in which a trader floods the market with fake orders by entering and quickly cancelling large buy or sell orders on an exchange, in order to fool other traders into thinking the market is poised to rise or fall. Though the tactic has long been used by some traders, regulators began clamping down on the practice only a few years ago.