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"We have made a lot of progress with the main point of the treaty, but we are not there yet where we want to be from a Swiss point of view. The demands of the Americans and the expectations of the Swiss are not yet in line and it will certainly need more talks," Widmer-Schlumpf said.

Earlier this month, the upper house of the Swiss parliament backed an amendment that would allow Switzerland to compel its banks to hand over American client data, even if authorities don't already know the client names; the lower house still has to approve it.

The amendment covers an existing tax treaty between the United States and Switzerland in which the Alpine country has agreed to hand over client data but generally only if the U.S. side already knows the client's identity.

Backed by court judges, federal prosecutors are issuing subpoenas -- official papers which compel the recipients to provide potentially damning evidence -- to United States taxpayers suspected of holding hidden accounts at Swiss and other offshore banks, according to criminal defence lawyers whose clients have received the papers.

The grand jury subpoenas are unusual in that they ask bank clients -- not the banks themselves -- to turn over to the authorities their bank account details since 2003, including statements with the highest annual balances. Taxpayers who refuse to comply potentially face a stark choice: be found in contempt of court and thus subject to civil or criminal fines and jail time, or disclose potentially incriminating evidence against themselves.

New US tactic for suspected Swiss bank tax cheats - Rueters

How does the IRS get on to you, you might wonder?

Increasingly, one important technique is the John Doe summons.  It blew the lid off the hushed world of Swiss banking in 2008 when a judge allowed the IRS to issue a John Doe summons to UBS for information about U.S. taxpayers using Swiss accounts.

Swiss law prohibits banks from revealing the identity of account holders, but the rest is history.

With a normal summons, the IRS seeks information about a specific taxpayer whose identity it knows. A John Doe summons allows the IRS to get the names of all taxpayers in a certain group. The IRS needs a judge to approve it, but recent IRS success may to lead to more.

A John Doe Summons is ideal for pursuing investors in tax shelters, account holders at financial institutions, attendees at an event, donors of real estate, etc.

After sniffing out American taxpayers with UBS accounts, the IRS did the same with HSBC in India. But the IRS tells its own examiners to use a John Doe Summons only after trying other routes. The IRS Manual says it may be possible to obtain taxpayer identities without issuing a John Doe summons.

IRS Summons? Meet John Doe - Forbes

A standard tool of defense in collections cases has been that the Internal Revenue Service cannot pierce state law entities such as a Single Member LLC.