Jamie Dimon’s email response was direct and to the point. “I approve,” he wrote to an oversight body within his enormous bank, JPMorgan Chase, thereby giving his blessing to an increase in the amount of risk the institution could shoulder. He also approved a change in the way a key trading unit was assessing threats of trouble in its then-burgeoning portfolio.

That email — released late Thursday as part of a 300-page Senate report probing how and why Dimon’s bank managed to lose $6.2 billion on derivatives trading — now appears to tie the chief executive directly to the disastrous decision-making at issue.

The trading occurred in the London offices of a bank unit known as the chief investment office, which was officially tasked with hedging against losses in JPMorgan’s broader positions. The Senate report portrays the CIO as a locus of reckless speculation, asserting that executives with oversight ordered the manipulation of a key risk model to make its activities appear conservative.

According to the report, for four days beginning on January 20, 2012, the CIO’s trading exceeded the bank’s own acceptable risk measures by one metric. But the bank did not conduct an internal review. Instead, JPMorgan temporarily raised its risk limits. Then, on January 27, the CIO tweaked its very measure of risk: The change immediately cut in half the CIO’s supposed risk, pushing it well below the bank’s limit.

According the report, this effective manipulation enabled the CIO to triple the size of the portfolio that eventually produced its headline-grabbing derivatives losses in the first three months of 2012.

The email trail included in the report suggests that responsibility for this change goes all the way up to Dimon.

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