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Ms. Lerner said the practice was initiated by low-level workers in Cincinnati and was not motivated by political bias. After her talk, she told the Associated Press that no high-level IRS officials knew about the practice. She did not say when the political targeting was discovered.

About 75 groups were inappropriately targeted. None had their tax-exempt status revoked, Ms. Lerner said.

Several tea party-affiliated groups complained last year the IRS had unfairly targeted them. Mr. McConnell, in a July speech to the conservative American Enterprise Institute, accused the tax agency of leaking sensitive information from conservative groups to the president’s re-election campaign.

The Tea Party Patriots, which calls itself as the nation’s largest tea party organization, rejected the IRS’s apology and demanded the immediate resignations of all of those involved in the “deliberate harassment” of tea party groups.

“The IRS has demonstrated the most disturbing, illegal and outrageous abuse of government power,” said Jenny Beth Martin, the group’s national coordinator.

Ms. Martin also demanded President Obama apologize and called for Congress to investigate.

This article is based in part on wire service reports.