WASHINGTON — An 18-month congressional investigation into the Internal Revenue Service’s mistreatment of conservative political groups seeking tax exemptions failed to show coordination between agency officials and political operatives in the White House, according to a report released on Tuesday.

The I.R.S. has admitted that before the 2012 election it inappropriately delayed approval of tax exemption applications by groups affiliated with the Tea Party movement, but the I.R.S. and its parent agency, the Treasury Department, have said that the errors were not motivated by partisanship.

Republican lawmakers, dismissing the Obama administration’s denials, have suggested that the delays were not only politically motivated but also orchestrated by the White House.

Some of the most strident comments have come from Representative Darrell Issa, Republican of California and the chairman of the House Oversight Committee, which has issued subpoenas to compel testimony from administration officials and held a series of tumultuous hearings on the I.R.S. scandal.