By LAURA SAUNDERS
Oct. 22, 2015 5:33 a.m. ET

Some Swiss banks loaded funds onto untraceable debit cards. At another, clients who wanted to transfer cash used code phrases such as “Can you download some tunes for us?” One bank allowed a client to convert Swiss francs into gold, which was then stored in a relative’s safe-deposit box.

Dozens of Swiss banks have been spilling their secrets this year as to how they encouraged U.S. clients to hide money abroad, part of a Justice Department program that lets them avoid prosecution. It is part of a broader U.S. crackdown on undeclared offshore accounts that has ensnared big Swiss banks such as UBS Group AG, but has received scant attention because it mostly involves little-known firms and relatively small fines.

A Wall Street Journal analysis of Justice Department documents from more than 40 settlements with these banks provides a rare window into foreign firms’ tax-haven techniques and their myriad methods of keeping clients’ accounts under wraps. The offenders range from international banks to small-town mortgage lenders, which together helped secrete more than 10,000 U.S-related accounts holding more than $10 billion, according to the analysis.

“Helping Americans conceal assets from the IRS was a big business for many sizes and types of firms in Switzerland, and now we’re seeing how extensive it was,” said Jeffrey Neiman, who led the Justice investigation of UBS in 2009 that pierced the veil of Swiss-bank secrecy. He is now at law firm Marcus Neiman & Rashbaum LLP in Fort Lauderdale, Fla.

The firms that have admitted to the misconduct have paid a total of more than $360 million to resolve the cases and avoid criminal charges. Lawyers for U.S. account holders and Swiss banks estimate that 40 other firms in this program are in talks with the Justice Department.

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“Banks large and small are naming individuals and firms that helped U.S. taxpayers hide foreign accounts and evade taxes,” said acting Assistant Attorney General Caroline Ciraolo. “It is now clear that asset-management firms, investment-advisory groups, insurance companies and corporate service providers—not just banks—facilitated this criminal conduct.”

More than 54,000 U.S. taxpayers with undeclared accounts have paid more than $8 billion to the Internal Revenue Service to resolve their cases and avoid criminal prosecution.

The following methods used by the Swiss firms were laid out in detail in signed statements from settlement documents and disclose misconduct dating back to 2008, although some wrongdoing had occurred for decades.

Most firms said they provided clients with numbered or code-name accounts with identities known only to a few employees. The banks also held mail destined for the U.S., in part to calm clients who were afraid the U.S. government might go through it. One client of Bank Sparhafen Zurich AG told a manager the bank’s hold-mail fee was “cheap insurance against having my dealings with you come to the attention of the government revenue authorities.”

Many banks also helped U.S. taxpayers further cover up their ownership of accounts in sham trusts and foundations in tax havens such as Liechtenstein, the British Virgin Islands and Panama.

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To give U.S. clients ready access to funds, some banks issued prepaid debit cards that were hard to trace. BSI SA of Lugano, which is one of the largest private banks in Switzerland, said some clients in need of cash sent emails with coded phrases like “gas tank still running on empty.”

Firms also accommodated clients who were fearful of U.S. money-laundering rules that require scrutiny of transfers above $10,000. St. Galler Kantonalbank AG, a regional bank controlled by the Swiss canton of St. Gallen, said it allowed nine U.S. customers to make a total of $3 million of withdrawals in amounts less than $10,000.

The bank in 2009 allowed one client who closed an account holding 1 million francs, which is about $1 million today, to use 400,000 francs to buy gold. The gold and some remaining francs were then put in a safe-deposit box at the bank in the name of a non-U.S. relative of the client, the bank said.

Many banks accepted accounts from clients fleeing large Swiss banks that were under pressure from U.S. authorities for practices the smaller firms are now admitting to.

Some of them saw the U.S.’s actions against the large firms as an opportunity. In late 2008, when UBS was facing possible criminal charges, Migros Bank AG, a Zurich-based firm that once mainly served middle- and working-class Swiss clients, convened a working group to weigh whether to do business with U.S. clients. They decided it was a profitable business worth expanding.

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In 2009 and 2010, Migros accepted more than 75 new U.S. accounts, many of them from larger Swiss banks. In late 2010, an executive said that as a medium-size Swiss bank with mostly Swiss customers, the bank wouldn’t “be in the crosshairs for the U.S. authorities.”

Yet, by the spring of 2012 as the U.S. pursuit of Swiss banks widened, Migros was pressuring U.S. clients to declare accounts to the IRS. At the same time, it allowed them to withdraw or transfer funds in ways designed to conceal them to the IRS, including transferring assets to secret accounts at other Swiss banks.

Firms can reduce penalties by taking steps such as turning over account information and by proving that clients have informed the IRS about their accounts. The penalties for individual clients can be as high as half the account’s value.

Lawyers for three of the Swiss banks said they are complying with terms of the settlements, and the lawyer for St. Galler Kantonalbank couldn’t be reached for comment. All firms that have signed agreements with the Justice Department have pledged not to violate U.S. laws in the future and to cooperate with U.S. authorities in efforts to combat tax evasion.

UBS paid $780 million and admitted wrongdoing in a 2009 settlement with U.S. authorities to avoid criminal charges. Credit Suisse Group AG pleaded guilty to criminal charges in 2014 and agreed to pay $2.6 billion.

Lake Leman in Geneva. Firms that have admitted to misconduct have paid a total of more than $360 million to resolve the cases and avoid criminal charges. ENLARGE
Lake Leman in Geneva. Firms that have admitted to misconduct have paid a total of more than $360 million to resolve the cases and avoid criminal charges. PHOTO: DENIS BALIBOUSE/REUTERS
The Justice Department said it hopes to reach settlements in this program with other Swiss firms by year-end. Lawyers for U.S. account holders and Swiss banks familiar with Justice Department efforts said that six to eight larger banks remain in separate talks with U.S. authorities.

Meanwhile, U.S. officials said they are mining the extensive data uncovered by the program, including the destination of funds transferred out of Swiss accounts, to pursue leads around the world.