In an early morning chat, three senior currency traders at some of the world's biggest banks weighed the pros and cons of admitting a fourth member to their private instant-message group.

The traders — from Citigroup Inc., JPMorgan Chase & Co. and UBS AG — had worked together for years to manipulate the $5.3 trillion-a-day currency market by sharing details of client orders and coordinating trading strategies, two people with knowledge of a global investigation into the foreign-exchange market said last year. While adding a new recruit would bolster their strength, they worried he couldn't be trusted to put the group's interests ahead of his firm's.

“Will he tell the rest of desk stuff,” Richard Usher, JPMorgan's chief London-based dealer, wrote in the chat published Wednesday by the U.S. Commodity Futures Trading Commission. “Or god forbid his nyk,” he said, referring to the New York trading desk.