“Mistakes were made, but they were in no way due to any political or partisan motivation. We are — and will continue to be — dedicated to reviewing all applications for tax-exempt status in an impartial manner,” wrote Miller in an op-ed in USA Today.

Miller said that the agency had sought a centralized system to handle the surge in groups requesting tax-exempt status, saying such applications for 501 (c)(4) status doubled from 1,591 to 3,398 between 2010 and 2012.

“Because the law limits and in some cases prohibits political intervention by exempt organizations, the IRS must carefully review applications based on the facts of each case. While centralizing cases for consistency made sense, the way we initially centralized them did not,” acknowledged Miller.

“The mistakes we made were due to the absence of a sufficient process for working the increase in cases and a lack of sensitivity to the implications of some of the decisions that were made,” he added.

The IRS said last week that low-level employees in its Cincinnati office tasked with reviewing the applications singled out groups with the words “Tea Party” and “patriot” for heightened scrutiny.

The revelations have sparked anger in Washington, with President Obama calling the actions “outrageous,” and the Ways and Means committee setting a hearing for Friday.

Senate Minority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellTeacher defeats Kentucky state House majority leader in GOP primary Conservatives leery of FBI deal on informant Lobbying world MORE (R-Ky.) and Sen. Marco RubioMarco Antonio RubioRepublicans think Trump is losing trade war The Hill's 12:30 Report — Sponsored by Delta Air Lines — Trump meets South Korean leader as questions linger about summit with North Senators demand answers on Trump’s ZTE deal MORE (R-Fla.) on Monday also called for Miller’s resignation.

A report in The Washington Post on Tuesday also revealed that the scope of the scandal could be wider than believed, with the paper finding that officials in the IRS Washington, D.C., office and two California offices also targeted Tea Party groups with questionnaires about their donors and voter outreach efforts.

In his op-ed, Miller though said the agency had corrected the problems that led to the earlier missteps.

“The new procedures we have implemented ensure the mistakes we made won't be repeated. We want to stress that our employees — all career civil servants — will continue to be guided by tax law and not by partisan issues,” said Miller.

“The IRS remains committed to protecting the integrity of our nation's tax system,” he added.