Ina Drew, the firm's former chief investment officer overseeing trading strategy, says the other executives failed to control risk out of the London office and that prevented her from controlling the losses.
Drew resigned last spring after the trading loss came to light. The comments to the Senate Permanent Subcommittee on Investigations were her first since the resignation.
The hearing occurred a day after the Senate panel issued a report that ascribed widespread blame to key executives at the firm. The report said they ignored growing risks and hid losses from investors and federal regulators
Executives at JPMorgan understated the trading losses to federal examiners by hundreds of millions of dollars and dismissed questions raised about the trading risks, according to the report.
The report also suggested that key executives, including CEO Jamie Dimon, were aware of huge losses at the bank, even while they were downplaying the risks publicly.
On Thursday, JPMorgan acknowledged it made mistakes but rejected any assertions that it concealed losses or risks. A spokesman declined to comment directly on the accusation that Dimon knew of the trading loss in April.
In April, news reports said a trader in JPMorgan's London office known as "the whale" had taken huge risks that were roiling the markets. Dimon immediately dismissed the reports as a "tempest in a teapot" during a conference call with analysts.
But in May, Dimon acknowledged that the bank had lost roughly $2 billon. And during testimony to a separate Senate panel in June, Dimon said the bank showed "bad judgment," was "stupid" and "took far too much risk."
The figure was later revised to more than $6 billion.
The loss came less than four years after the 2008 financial crisis and hurt the reputation of a bank that had come through the crisis known for taking fewer risks than its competitors. Three employees in the London office were fired - two senior managers and a trader. It also led to Drew's resignation.