After a bit of back and forth, Mr. Braunstein said: “No, that is not acceptable practice.”

Not acceptable, perhaps, but that is what occurred, as the Senate report shows. Normal practice at the bank and across the industry is to value these kinds of derivatives at the midpoint between the bid and offer prices available in the market. But in early 2012, as it became apparent that JPMorgan’s big trades at the chief investment office were going bad, the bank began valuing the portfolio well outside the midpoint. This reduced its losses.

For example, in January 2012, the portfolio valuations hewed closely to the midpoint on all but 2 of the 18 measures, the Senate investigators found. A month later, 5 of the 18 valuation measures deviated from the midpoint. In March, however, all 18 deviated, and 16 were at the outer bounds of price ranges. In every case, the prices used by the bank understated its losses.

While these valuation shifts were taking place in the chief investment office, JPMorgan’s investment bank officials continued to mark their identical positions using the midpoint value.

RISK limits, intended to protect the bank from losses, were also routinely breached at JPMorgan Chase, the report found. From late 2011 to the first quarter of 2012, Senate investigators saw a huge jump in the number of risk-limit breaches — to more than 170, from 6. Then, in April 2012 alone, risk limits were exceeded 160 times.

“Should someone have investigated the risky trading activities that triggered all these breaches?” Mr. Levin asked. Yes, but no one did, the report concluded. The risk limits were either ignored or modified to make the portfolio look better.

JPMorgan Chase has repeatedly said it made mistakes and has changed its policies.

The Senate report also raises questions about the rigor with which JPMorgan conducted its own investigation into the trading loss. That report was published in January.

Mr. Levin cited the internal report’s failings in his questioning of Michael Cavanagh, a task force member and co-C.E.O. of JPMorgan’s corporate and investment bank. “You just told us that shifting pricing practices to minimize losses is not acceptable,” Mr. Levin said. “Did you say that in your report? Did you say that’s what happened?”