How did Dutch authorities find so much data on tax cheats linked to Credit Suisse? A little help from a powerful friend – one which has extracted billions from Swiss banks in recent years.

Much of a Dutch-led, six-country tax probe that has ensnared Credit Suisse remains mysterious.

The Swiss bank has surfaced at the center of a criminal investigation by the Netherlands, Britain, Germany, France, Australia and Austria into tax evasion. Several people have been arrested in Holland, a British woman was arrested last week, and assets such as luxury properties, valuable cars, jewelry, and even a gold ingot were seized.

To complete their puzzle, Dutch authorities won a crucial chunk of evidence from U.S. investigators – the same investigators who criminally investigated Credit Suisse over helping wealthy Americans to cheat on their taxes.

Amex Ruling

In fact, U.S. authorities went to the trouble of getting a court ruling to force American Express to cough up years of credit card data on Dutch residents with debit or credit cards linked to bank accounts outside the Netherlands.

The move is one of the first times the U.S. has wielded its considerable prosecuting power on behalf of another nation. It also indicates that the U.S. is on board for a more global fight against tax evasion, and not just on American shores.

«In fighting international tax evasion, the IRS recognizes hidden offshore accounts are a problem other nations face as well,» IRS Commissioner John Koskinen said in a statement.

European Raids

Credit Suisse's offices in several cities – Amsterdam and Paris – were searched, reopening a potentially large new probe just as the bank finds its footing after a wide-ranging restructuring. Until now there is little indication that the bank itself, as opposed to clients, is in the crosshairs of investigators.

The bank paid $2.6 billion in fines and penalties nearly three years ago and pleaded guilty to criminal charges to settle a long-running U.S. tax investigation.

One day later, a high-ranking prosecutor disclosed that Credit Suisse had hindered the probe: the bank initially failed to remedy the problem, dragging its feet on investigating internally by not interviewing those involved or introducing policies to store relevant work documents.

CS on Tenterhooks

A spokeswoman for Credit Suisse wasn't immediately available for comment. The bank isn't being investigated in Europe yet, but the Americans' involvement is not good news for Credit Suisse.

The bank is already on tenterhooks because it is reportedly being re-investigated by the very same U.S. officials over allegations it continued doing the same illicit banking in another unit of the bank.

Dan Horsky, a wealthy Israeli-American retired business professor with ties to Switzerland, has sparked the renewed investigation. The 71-year-old Horsky hid several hundred million at Credit Suisse before turning U.S. informant, delivering information about banks and relationship managers.