"All I can say is that we have opened a preliminary investigation," Robert Mueller said in response to questions at an FBI oversight hearing on Capitol Hill.
It remains unclear what potential crimes the FBI may be investigating, and Mueller declined to answer questions on that topic Wednesday, saying only that the agency had opened a preliminary investigation. JPMorgan's Dimon gets his $23 million pay package
Media reports also indicate the Securities and Exchange Commission has launched an investigation of its own, a move that would be expected following a trading loss of the magnitude suffered by JPMorgan.
JPMorgan (JPM, Fortune 500) shares were up slightly in trading on Wednesday, but are still down more than 11% over the previous five trading sessions.
The bank's chief executive, Jamie Dimon, has repeatedly apologized in recent days for the bank's actions, which he called "stupid."
"This should never have happened. I can't justify it. Unfortunately these mistakes were self inflicted," Dimon told shareholders at the bank's annual meeting on Tuesday.
JPMorgan has formed its own committee to investigate the trades, and on Monday it announced the retirement of Ina Drew, the firm's chief investment officer.
A statement issued by the company said Drew made the decision to retire, a move that was widely expected after the company disclosed the unit she managed had suffered a major loss.
Net losses from the trades, after factoring in other securities gains, are expected to exceed $800 million by the end of the second quarter. And losses could increase depending on market conditions and the bank's actions moving forward.