The outgoing head of the IRS told Congress that the US tax agency made \"foolish mistakes\" in targeting conservative groups, but insisted the action was not politically motivated. Lawmakers hammered Steven Miller, who resigned this week at the request of President Barack Obama, over abuse of power at the agency that Democrats and Republicans alike criticized as outrageous and unacceptable. The Internal Revenue Service acknowledged last week that in 2010 employees subjected conservative groups, including those with \"Tea Party\" and \"Patriots\" in their names, to increased scrutiny when they applied for non-profit status. Miller, at a tense hearing of the House Ways and Means Committee, apologized for what he acknowledged were \"mistakes\" and \"horrible customer service\" provided by the IRS. \"Even the perception of partisanship has no place at the IRS,\" Miller said. \"I do not believe that partisanship motivated the people who engaged in the practices described\" in an internal report on the abuse. \"Foolish mistakes were made by people trying to be more efficient in their workload selection.\" Miller faced a four-hour grilling from lawmakers, including Republican committee chairman Dave Camp, who was angry that the IRS never told Congress that top officials, including Miller, knew about the abuse in May 2012. \"In fact, we were repeatedly told no such targeting was happening,\" Camp told Miller. \"That isn\'t being misleading, that is lying.\" Camp demanded that Miller tell Congress \"who started the targeting, who knew, when did they know, and how high did it go?\" Obama said Thursday he had no prior knowledge of the abuse, and Miller told the hearing he \"absolutely\" did not contact the White House when he first learned of the burdensome scrutiny of the conservative groups last May. Miller\'s successor was ordered to launch a thorough review of the agency in the wake of the scandal. US Treasury Secretary Jacob Lew gave Daniel Werfel one month to report progress in revamping the organization \"in an effort to restore public confidence in the IRS and ensure the organization is providing excellent and unbiased service to the taxpayer,\" a Treasury official said. Some Republicans have seized on the scandal -- coupled with the revelation that the Justice Department secretly seized several journalists\' phone records -- as an example of big-brother government run amok. They are also probing possible links to the White House in an attempt to draw the Democratic president into the controversy. Lawmakers like Republican Kevin Brady demanded to know who exactly was responsible for the wrongdoing. \"I don\'t have names for you, Mr Brady,\" Miller said. While he acknowledged the that excess scrutiny was \"inappropriate,\" he stressed: \"It\'s my belief that what happened here wasn\'t illegal.\" The Treasury\'s Inspector General for Tax Administration Russell George, who investigated the wrongdoing, concurred, saying that while targeting such groups is unusual, \"it is not illegal.\" But several lawmakers including Paul Ryan, last year\'s Republican vice presidential nominee, insisted Miller withheld information from Congress last July by not disclosing the abuse he had learned about two months earlier. Miller bristled at Ryan\'s accusation. \"I did not mislead the committee. I stand by my answer then; I stand by my answer now,\" he said. Congressional fury was bipartisan in the hearing, where leaders of some Tea Party groups sat in the audience. Senior Democrat Sander Levin called for the resignation of Lois Lerner, the senior IRS official who acknowledged the wrongdoing a week ago, days after telling a congressional committee nothing of the abuse. Miller described a process of \"triage\" in dealing with 70,000 applications for non-profit status that flooded in after the Supreme Court ruled in 2010 it is legal for companies or organizations to spend money on political activity. The understaffed department studying the applications began \"centralizing\" the groups, Miller said. While he criticized the methods, he defended the close scrutiny, saying \"politics is an area where we always asked more questions, as we are obligated by law to do.\" Seething Democrat Charles Rangel said the IRS\'s integrity, as well as that of the president, was on the line. \"People are losing confidence in our government,\" he told Miller, \"and I hope that you feel the same sense to find out what caused this... and help us to restore the confidence that Americans should have in their government.\"