U.S. prosecutors announced criminal charges against two former J.P. Morgan Chase & Co. traders who were accused of hiding losses on runaway bad bets that cost the bank more than $6 billion.

The charges are the U.S. government's most aggressive move yet to hold someone responsible for the mess named after the "London whale," who engineered the bets and worked closely with the two former J.P. Morgan traders—though he avoided prosecution and helped investigators build a case against them.

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