Levin described the experience of one account holder: “He was guided into an elevator remotely controlled, with no buttons, no floors. The meeting took place not at a banker’s office, but in a separate bare, conference room. All of these being a dramatic demonstration to the client of the secrecy that Credit Suisse practiced.”

The committee details three ways that it estimated the amount of undeclared assets held by the bank’s U.S. clients. It concluded that “the vast majority” of 22,000 accounts opened for U.S. customers -- 85 percent to 95 percent -- may have been hidden from the IRS.

At its peak from 2001 to 2008, the bank held as much as $12 billion in assets for U.S. customers, according to the report, meaning that about $10 billion was undeclared. The indictment in 2011 said the amount was $4 billion.

“The amount of assets associated with the bank’s undeclared Swiss accounts are likely significantly greater than the amount cited by DOJ in indictment,” according to the report.

UBS Case

The U.S. crackdown on offshore tax evasion accelerated after 2009, when UBS AG, the largest Swiss bank, avoided prosecution by paying $780 million, admitting it fostered tax evasion and turning over the names of 4,700 account holders.

Prosecutors charged more than 70 taxpayers and three dozen offshore bankers, lawyers and advisers. More than 43,000 Americans avoided prosecution by voluntarily disclosing their accounts to the IRS and helping investigators.

The report criticizes prosecutors for failing to go to U.S. courts more often to file civil summonses seeking names of unidentified clients suspected of tax crimes, as it did in the UBS case. It also faults the Justice Department for failing to enforce grand-jury subpoenas, such as one it issued to Credit Suisse in 2011.

Instead, the U.S. has made requests for client names through a tax treaty with Switzerland and has failed to overcome Swiss objections based on that nation’s laws protecting account secrecy.

Emily Pierce, a Justice Department spokeswoman, defended the actions of U.S. prosecutors.