An 18-month-long Congressional investigation into whether the Internal Revenue Service (IRS) colluded with the White House in the mistreatment of conservative political groups concludes there was no evidence.

A report by the House Oversight Committee, 226 pages in length, said despite the IRS admitting that before the 2012 election that it delayed tax exemption applications by groups affiliated with the Tea Party movement, the delay was not the result of partisanship.

Republican lawmakers during the investigation maintained the delay was politically motivated and under instruction from the White House despite the Obama administration's repeated denials.

The investigation involved several contentious hearings, with subpoenas being issued to compel testimony from administration officials. During the hearings, the chairman of the House Oversight Committee, Rep. Darrell Issa (R-Calif.), accused the IRS of a Watergate-style cover-up and administration officials of obstructing his investigation.

The report said when IRS employees evaluated the applications, they wondered whether the groups' activities were "good" non-profit activities or merely "emotional" propaganda with "little educational value."

The investigation involved millions of documents and dozens of interviews with Obama administration officials. The report said that "IRS officials failed to limit their professional judgment to enforcing the tax does and instead inserted their own beliefs and judgments into federal matters to influence the outcomes and decisions."

According to the report, conservative organizations were singled out because of their political beliefs. One IRS agent wrote about an organization applying for 501(c) 4 status that donated to other organizations that engaged in political activity: "It appears that the org is funneling money to other orgs for political purposes. However, I'm not sure we can deny them because, technically, I don't know that I can deny them simply for donating to another 501(c) 4 ... Any thoughts or feedback would be greatly appreciated." Another agent responded, "I think there may be a number of ways to deny them. ...This sounds like a bad org ... This org gives me an icky feeling."

Rep. Issa won't continue as committee chair in the new Congress, being replaced by Rep. Jason Chaffetz, (R-Utah), who said the investigation is not over as 30,000 IRS emails have been recovered that were thought to have been lost in computer crashes.

Rep. Elijah E. Cummings (D-Md.) said the IRS inquiry was comparable to Sen. Joseph R. McCarthy's investigation of suspected communists in the 1950s, and was sharply critical of the report.

"It is revealing that the Republicans -- yet again -- are leaking cherry-picked excerpts of documents to support their preconceived political narrative without allowing committee members to even see their conclusions or vote on them first," Cummings, the ranking Democrat on the committee told The New York Times.