When reports emerged in the first week of April that a JPMorgan Chase trader in London was making bets so big they were distorting a little-known market in credit derivatives, an embedded regulator approached executives.

Jamie Dimon

The senior supervisor of the Federal Reserve Bank of New York, one of roughly 40 Fed officials who work inside the bank, was told that senior management was not overly concerned about the reports, according to a person briefed on the matter.