Oversight Committee Chairman Darrell Issa turned up the heat early when he accused Jennifer O'Connor, a former IRS attorney who now works at the White House, of being a "hostile witness".
O'Connor denied the charge and Issa later backtracked to say she is "non-cooperative" instead of hostile.
O'Connor, who worked at the IRS from May 2013 to November 2013, helped the IRS gather documents related to Congressional investigations focusing on alleged targeting of conservative political groups.
Issa has said O'Connor's testimony was necessary because during that period she had a primary role in gathering the documents to be presented to the committee.
Democrats have said it is purely a political ploy.
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