Two senior tax officials in the United States have denied that extra scrutiny given to conservative groups seeking tax exemptions before the 2012 election, was motivated by partisan bias.
Acting Internal Revenue Service head Steven Miller told the House Ways and Means Committee on Friday the "mistakes" were an effort to handle a flood of applications.
Mr Miller and another top IRS staff member have resigned over the matter, which the FBI is investigating.
The practice of extra screening began in response to a Supreme Court ruling that loosened campaign finance rules, Mr Miller said.
A list of keywords that flagged conservative groups for extra review had merely been put together by civil servants trying to work more efficiently.
But during questions, Mr Miller said the agents did not use search words associated with liberal causes - like "progress" or "organising".
An investigation by Treasury inspector general J Russell George found that 296 groups had been subjected to additional auditing. Many of the applicants faced considerable delays in obtaining tax-exempt status.
The BBC reports Mr George told the committee that he had not seen evidence that IRS officials were under political pressure to target conservative groups.
Committee chair Dave Camp (Michigan, Republican) said the affair appeared to be part of a "culture of cover-ups" in the IRS.
"It seems like the truth is hidden from the American people just long enough to make it through an election," Mr Camp said.
The IRS is an independent agency within the Treasury Department.
In addition to Mr Miller, IRS commissioner Joseph Grant said on Thursday that he was stepping down within a month.