NEW YORK—JPMorgan Chase said Friday that its traders may have tried to conceal the losses from a soured bet that has embarrassed the bank and cost it almost $6 billion -- far more than its CEO first suggested.

The bank said an internal investigation had uncovered evidence that led executives to "question the integrity" of the values, or marks, that traders assigned to their trades.

JPMorgan also said that it planned to revoke two years' worth of pay from some of the senior managers involved in the bad bet, and that it had closed the division of the bank responsible for the mistake.

"This has shaken our company to the core," CEO Jamie Dimon said.

The bank said the loss, which Dimon estimated at $2 billion when he disclosed it in May, had grown to $5.8 billion, and could grow larger than $7 billion if financial markets deteriorate severely.

Dimon said the worst appeared to be behind the bank, and investors seemed to agree: They sent JPMorgan stock up 6 percent, making it the best performer in the Dow Jones industrial average.

Daniel Alpert, a founding managing partner with the New York investment bank Westwood Capital Partners LLC, said the bank and Dimon appeared to have learned from the crisis.

He said Dimon now realizes how complex and difficult to manage the bank is, will be more diligent in the future and probably won't be the crusader he has been against some proposed financial regulation.

"Did it cost shareholders a few bucks? Yup," he said. "But it was a non-horrible way of learning the lesson, in the sense that the entire institution didn't burn down, the lesson's been taught and Dimon seems ready to take it."

For his part, Dimon concluded: "We are not proud of this moment, but we are proud of our company."

The investigation, which covered more than a million emails and tens of thousands of voice messages, suggested traders were trying to make losses look smaller, the bank said.

The revelation could expose JPMorgan to civil fraud charges. If regulators decide that employee deceptions caused JPMorgan to report inaccurate financial details, they could pursue charges against the employees, the bank or both.

The Justice Department, the Securities and Exchange Commission and other regulators, including one in Britain, are looking into the loss. The Justice Department and SEC declined comment.

JPMorgan could not necessarily hide behind the actions of its employees. Regulators could decide that its oversight or risk management contributed to the problematic statements.