The investigation now includes full interviews with 15 employees — six Republicans, three Democrats, and six "unaffiliated" workers — the memo says.
"Despite an extremely aggressive investigation involving thousands of documents and more than a dozen interviews of IRS employees, the overwhelming evidence before the committee reveals no political motivation or White House involvement in this process," the memo reads.
Instead, the IRS employees in the tax service's Cincinnati office sought guidance on how to process organizations applying for tax-exempt status in order to treat applicants consistently, the memo said.
The Cincinnati group manager in charge of employees handling political advocacy cases from May 2010 through February 2013, a self-identified Republican, echoed past claims that "tea party" cases were singled out for evidence of political advocacy.
"Review of the application showed that [there] wasn't the precedents there to help guide through the application process," the manager said. "So ... we reached out to our Washington office for guidance."
A Republican tax law specialist in Washington, D.C. who handled tea party applications and drafted a guide sheet on how to handle political advocacy cases also said there was no White House involvement or bias against tea party groups.
She described as "kind of laughable" the idea of having a particular group in the D.C. office involved in advocacy cases based on an attempt to target President Obama's political enemies.
"This is purely cases that, unfortunately, Cincinnati didn't have enough guidance on," she said. "That [tax-exempt] area is a very, very difficult area, and there's not much guidance. And so the lingering length of time, unfortunately, was just trying to apply the law to the specific facts of each case."
The GOP-led House committee recently approved a resolution asserting that the woman at the center of the IRS fiasco waived her Fifth Amendment rights against self-incrimination when she came before the committee in May.
Lois Lerner, director of tax-exempt organizations for the agency, began her testimony by denying that she acted improperly and then immediately invoked her right not to testify against herself.
Ms. Lerner tried to stop the Cincinnati field office's targeting of tea party and other right-leaning groups by directing specialists in 2011 to broaden their criteria so that it did not appear partisan, according to an audit by J. Russell George, the Treasury inspector general for tax administration. Yet the audit says the behavior resurfaced and had to be addressed once more in May 2012.
Ms. Lerner apologized at a May event with the American Bar Association for burdening the conservative groups from early 2010 to May 2012, an admission that set off a firestorm on Capitol Hill.
The announcement — staged through a prearranged question from the audience — was made days before the inspector general released the audit that confirmed Republican lawmakers' suspicions in 2012 that conservative groups had been targeted by tax investigators.
Since then, however, Democrats have pushed for Mr. George to return to the committee and testify after it was discovered that an array of terms — not just "tea party or "9/12 project," for example — were on so-called BOLO ("be-on-the-lookout") lists.
Mr. George is among those scheduled to testify before the committee Thursday.
The Washington Times reported Monday that Mr. George informed Sen. Charles E. Grassley, Iowa Republican, that for the first time, confidential confidential tax records of several political candidates and campaign donors were improperly scrutinized by government officials, but the Justice Department has declined to prosecute any of the cases.