It's pretty clear from this passage in the middle:

>As the United States sought to cement its newfound dominance in the world, counterposed against that of the Soviet sphere of influence, it entered a period of rigorous political control of the academy. The epithet usually applied to this era of persecution and paranoia is “McCarthyism,” but the phenomenon is wider than the term suggests. The persecution extended well beyond McCarthy’s notorious House Un-American Activities Committee (HUAC). The chief motor of the surveillance and persecution was, in fact, J. Edgar Hoover’s FBI. Both Carnap and Reichenbach were subjected to surveillance and harassment from the FBI. Since the FBI regularly intercepted letters, anyone could be drawn into its realm of suspicion (they had got onto Carnap by reading the correspondence of another Vienna Circle member, Philipp Frank). Mere association with the whiff of Communist ideas or activity could be enough to prompt warranted anxiety.

>The climate of fear operated according to a very simple logic. Academics who were suspected of being Communists were called before HUAC, or before various associated committees (the Rapp-Coudert Committee in New York, the Canwell Committee in Washington State). They were called as witnesses, but effectively they were defendants. If they were found guilty, either because they admitted to being or having been Communists, or by remaining silent, then dismissal followed, by means of the following straightforward argument, schematized by Victor Lowe in the pages of the Journal of Philosophy:

>>1) Professor X is a Communist.

>>2) A Communist has no respect for freedom of inquiry or for objectivity in teaching; to put it positively, he indoctrinates for the party line and the Soviet dictatorship.

>>Therefore 3) X is not fit to be a professor.

>In other words, the professor’s actual political views need not be examined, nor their teaching record.