At the same time the Internal Revenue Service was targeting tea-party groups, the tax agency took the unusual step of trying to impose gift taxes on donors to a prominent conservative advocacy group formed in 2007 to build support for President George W. Bush's Iraq troop surge.

The probe of the group, Freedom's Watch, began in the unit led by Lois Lerner, the IRS official already under scrutiny for her role in the more recent targeting of conservative groups.

While the IRS confirmed the existence of the gift-tax initiative in 2011, the identity of the group involved—as well as the affiliation of individual donors—remained a mystery.

Former officials of Freedom's Watch say they believe all five of the IRS audits involved donors to their group, based on conversations with IRS agents and donors at the time of the audits in 2011.

The IRS declined to comment, citing taxpayer confidentiality.