A US congressional panel grilled the deposed US tax chief on Friday, demanding answers about why and how revenue officials paid special attention to conservative groups seeking tax-free status, dpa reported.
Steven Miller, who resigned at the demand of the US president earlier this week, apologized on behalf of the Internal Revenue Service but denied suggestions there was political bias.
"I do not believe that partisanship motivated the people who engaged in the practices," Miller told the House of Representatives committee.
The charges of possible political bias were made in an inspector general's report that was released in recent days. The report found that tax officials used an "inappropriate" list of words like "tea party" and "conservative" to find groups they thought needed extra scrutiny before being granted nonprofit status.
The findings have provoked furor on both sides of the political aisle, and given added fodder for opposition Republicans against President Barack Obama as he deals with other hot button issues besieging the White House in recent days.
Republicans have charged that the bias served to help Obama's re-election in 2012. Miller denied that there was any involvement by the White House in setting up the screening criteria.
There were questions about whether conservative and Tea Party groups were the only ones singled out for scrutiny by tax authorities.
Democratic Congressman Richard Neal noted that tax authorities faced a huge influx of applications for tax-exempt status following the 2010 Supreme Court ruling that lifted restrictions on political spending by corporations and labour unions.
"There wasn't this rush because they wanted to join the Sisters of Mercy and Common Cause," Neal said. "It was in many instances to hide the donors."
Miller said that tax officials in charge of tax-exempt applications were overworked and made "foolish mistakes" in trying to devise systems to streamline their work.
"The listing described in the report, while intolerable, was a mistake and not an act of partisanship," Miller insisted.
Federal law allows non-profit groups to pursue public policy agendas but not to directly participate in election campaigns.
Attorney General Eric Holder, himself under fire over the seizure of telephone records from the Associated Press, has opened a criminal probe into the IRS actions. Obama has insisted that there is no need for a special prosecutor to investigate what he called "outrageous" actions at the IRS.