No one at the US tax agency will be charged in a two-year-old investigation stemming from allegations the agency improperly targeted conservative groups, the US Department of Justice said Friday.

The tea party - an informal group composed mostly of Republicans - and other conservative groups had accused the Internal Revenue Service of deliberately singling them out for extra scrutiny of their tax status after lengthy delays in their applications, dating back to 2010, for nonprofit, tax-exempt status.

Republicans called the scrutiny politically motivated and it erupted into a political scandal.

A US Treasury Department probe later confirmed the IRS had used inappropriate guidelines to try to identify groups that were political and thus not eligible for nonprofit status.

But Friday the Justice Department said that inappropriate behavior was not criminal.

"Our investigation uncovered substantial evidence of mismanagement, poor judgment and institutional inertia leading to the belief by many tax-exempt applicants that the IRS targeted them based on their political viewpoints. But poor management is not a crime," Assistant Attorney General Peter Kadzik said in a letter to Congress.

Based on that, the department is closing its investigation, he said.

Bob Goodlatte, chair of the US House of Representatives' Judiciary Committee, who launched a congressional probe into the matter in 2012, did not see Friday's Justice Department announcement as the final word on the matter.

"Even as far back as last year unnamed DOJ officials leaked information to the media suggesting that the Department did not plan to file criminal charges over the IRS's targeting of conservative groups," he said, suggesting the White House had "repeatedly and publicly undermined" the investigation.

He promised to raise the matter in a previously scheduled congressional hearing with Attorney General Loretta Lynch on oversight of DOJ Wednesday.