JPMorgan Chase & Co. trader Bruno Iksil, known as the London Whale because his bets this year were so large, has been a leviathan of a risk-taker since at least 2010, a person with knowledge of the matter said.

Iksil's value-at-risk, a measure of how much a trader might lose in one day, was typically $30 million to $40 million even before this year's buildup, said the person, who wasn't authorized to discuss the trades. Sometimes the figure, known as VaR, could surpass $60 million, the person said. That's about as high as the level for the firm's entire investment bank, which employs 26,000 people.

Investigators are examining how long senior executives knew about Iksil's swelling bets at the chief investment office before losses approached $2 billion. One focal point is why the formula used to calculate Iksil's VaR was altered early this year, cutting the reported risk by half. The change followed an internal analysis in late 2011 and was approved by top risk executives, said a person close to the bank. About the same time, half a dozen managers typically involved in such decisions moved to new jobs.