The Internal Revenue Service is investigating five donors for making contributions to the kind of nonprofit groups that have become popular for spending millions of dollars on political ads.
An agency spokeswoman says the IRS is trying to determine whether the donors owe federal gift taxes for the donations.
On Thursday, the IRS did not name the donors or the nonprofit groups to which they contributed.
But the agency's confirmation of an investigation could have a chilling effect on politically active groups that have become integral to campaigns for Congress and could play a big part in the 2012 presidential election.
At issue is whether contributors to the tax-exempt organizations have to pay the 35 percent gift tax on their donations. It is a murky area of the law.
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