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It is unusual that a number of your clients were visited. This is not ordinary IRS behavior. Audits involve advance mail notifications. It sounds like any one or combination of the following:
1.If these clients are invested in a common investment (something you can discern from your files) the IRS could be investigating the investment as a possible tax evasion scheme.
2.The IRS could have you as a target of their investigation for either suspicion of fraudulent returns, falsified investment/tax shelters, or investment promotions.
In any of these scenarios you should first determine what in the commonality of these clients. If it or even appears to be you then you are likely the target of their efforts. Get an attorney quickly and follow his/her advice to the letter. If the commonality is something with which you are not involved directly such as a common investment while you may not be their target you should at least speak with an attorney and be prepared to get him/her involved at a moments notice. Be careful what you say or write (in any form) in the meantime. There is nothing you can do to change the past but be cautious going forward.
There doesn't seem to be any commonality and the returns I've heard about so far are fairly conservative. One has large donations another small. 2 have unemployment. 2 have small businesses.