Calls for the criminal prosecution of bankers involved in rigging the global interest rate known as Libor are growing louder as government officials learn more about the scheme.

The European Commission on Wednesday proposed making the ma­nipu­la­tion of key financial benchmarks a criminal offense, responding to e-mails that show bankers on both sides of the Atlantic colluded in fiddling with Libor.

Short of instituting new legislation, U.S. authorities are pursuing criminal charges against those involved in the scheme, although the scope of the charges is unclear. Analysts say prosecutors are likely to hinge cases on securities fraud and conspiracy charges. But whether they stick is another matter.

When regulators and the Justice Department fined Barclays $450 million in June, prosecutors said the British bank was cooperating in criminal investigations of other financial institutions. Barclays acknowledged that some of its traders and senior executives manipulated rates for their own profit and to hide the bank’s distress from 2005 to 2009.

Regulators say similar practices may have occurred at other big banks that submit data to set the daily Libor rate — the benchmark for hundreds of trillions of dollars in loans and financial securities. The banks that set Libor include Citigroup, Bank of America and JPMorgan Chase.

If traders are found to have banded together to fix the rate, they would be in violation of the Sherman Act, a law that makes conspiracy to commit bank fraud punishable by up to 10 years in prison.

E-mails between traders released by Barclays could serve as the most damning evidence of a conspiracy. In one instance, a trader at an unidentified bank asked a Barclays employee for a lower Libor submission. At least 12 Libor requests made to Barclays employees came from traders who used to work at the British bank.

“The e-mails are a game-changer,” said Andrew Stoltmann, a securities lawyer in Chicago. “To the extent that you have multiple traders from multiple firms discussing how to manipulate Libor, this is a textbook example of a conspiracy.”

Former federal prosecutor Douglas Burns said that prosecutors could zero in on conspiracy to commit wire fraud or mail fraud, crimes that carry five years in prison per count — and that many counts could be charged.

Burns suspects securities fraud — deceptions that affect the price of a stock, bond or other financial instrument — will come into play. The Sarbanes-Oxley Act of 2002 made such acts criminal.

But he is skeptical that prosecutors can achieve any criminal convictions.

“A criminal case may be hard to make,” he said. “Banks and individuals have been charged in the past for lots of misconduct — cooking books, embezzlement, mortgage fraud . . . but the Libor rate-setting concept is a bit unusual.”

Some in the financial industry doubt that traders acted with criminal intent to manipulate the benchmark rate. Oswald Gruebel, former head of Swiss bank UBS, told Reuters on Wednesday that if bank traders acted with criminal intent, they would have done a better job of hiding the evidence. UBS, in March, was one of the first banks to acknowledge that it had been subpoenaed in an investigation of Libor fixing.

British and American regulators are coming under fire for their handling of inquiries, as e-mails and call transcripts show they had been aware of problems since 2007. European Commission Vice President Viviane Reding said she was not convinced that British regulators did all they could to stop the rate-fixing.

“I would have expected to see greater energy in reacting to calls — some made already over four years ago — for the elimination of incentives to misreporting,” she said. “This did not happen.”

The commission wants European Union members to criminalize any attempt to manipulate benchmark rates.

“We want to close any possible regulatory loopholes,” Reding said. “Criminal law can serve as a strong deterrence against any future manipulations.”