U.S. prosecutors are preparing criminal charges against a former supervisor and colleague of the JPMorgan Chase & Co. trader known as the “London whale” as they ramp up a probe of the supersize bets that backfired into more than $6 billion in losses, according to a media report Saturday.

The Wall Street Journal, citing unnamed people familiar with the matter, identified the potential defendants as:

n Javier Martin-Artajo, a Spaniard who worked in the London outpost of JPMorgan’s Chief Investment Office and was in charge of the team that made the giant wagers in corporate credit securities that piled up losses early in 2012.

n Julien Grout, the person responsible for placing daily values on the positions.

The U.S. attorney’s office in Manhattan has been conducting a criminal investigation of the trades, the Journal said.

Both men are believed to be in the U.K., so U.S. and U.K. officials likely would coordinate with each other on any arrest, according to the report, which could lead to a lengthy extradition process.

Both men worked with Bruno Iksil, the trader nicknamed the “London whale” for his outsize bets in these investments. Iksil isn’t expected to be charged, The Journal said. The potential arrests were reported earlier by the New York Times.