In the latest twist in a scandal that has tainted the reputation of the largest US bank and led to calls for greater oversight of its chief executive, Jamie Dimon, the main target of the investigation is Javier Martin-Artajo, who worked in London as the direct supervisor of Bruno Iksil, the trader who became known as "the London Whale," the sources said.
The United States is also looking at Julien Grout, Iksil's junior trader, according to one of the sources. Both sources spoke on condition that they not be otherwise identified as the investigation is ongoing.
It was reported on Thursday that Iksil, who earned his nickname after making outsized bets in a thinly traded derivatives market, is cooperating with the government and will not face any charges. His cooperation is essential to any arrest, the same sources said.               

The timing of the possible arrests, which would take place in London, was not clear, the sources said. US authorities plan to extradite the former employees to the United States, they said.
Lawyers for Martin-Artajo, Iksil and Grout did not immediately respond to requests for comment on Friday. All three employees have since left the bank.


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