The FBI's source on the Trump campaign during the 2016 presidential race is a veteran of three Republican administrations, The Washington Post reported.

Stefan A. Halper, a former professor emeritus at Cambridge University in England, was a secret informant on the Trump campaign during the FBI's investigation into Russia's interference in the 2016 presidential election, according to the newspaper.

Halper, 73, reportedly began work with the FBI's counterintelligence probe after the bureau obtained evidence of suspicious contacts between Trump campaign advisers and Russian operatives.

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The professor's career included positions in the White House as a domestic policy adviser for President Nixon and the chief of staff for President Ford, according to the Post.

Halper also reportedly served as a deputy assistant secretary of State in the Reagan administration. Halper and other aides were accused of spying on Jimmy Carter's campaign after Ronald Reagan was elected president in 1980, according to the Post, which added that the aides denied the accusations.

Cambridge told the newspaper that Halper taught international affairs and American studies until 2015 and drew on his extensive government contacts to host intelligence officials for seminars with students.

The professor's identity was revealed by multiple news outlets despite warnings from lawmakers and intelligence officials that outing the informant could raise legal and security concerns.

President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE has claimed that the informant was "embedded" in the campaign, though reports suggest Halper did not infiltrate Trump's ring of close advisers.