In 1948 he relocated to the United States, taking a position as a Professor of Physics at Syracuse University, roughly equidistant between Buffalo and Albany in Upstate New York. Here he embarked on research in nuclear physics and cosmic radiation. In 1953 he became a member of the New York Academy of Sciences. In the age of McCarthyism, Sitte was the object of official suspicion during his years in the United States, both because of his left-wing politics in the 1930s and because of the contacts he maintained with Czechoslovak communists whom he knew from his time in Buchenwald, and whom he continued to visit even after the Communist take-over in 1948. It was at the instigation of the FBI that in 1953 Sitte's residence permit was not renewed, and he accordingly relocated again, this time to Brazil. He took a teaching post as a visiting professor at the University of São Paulo. Shortly afterwards his inclusion on a list of security risks was confirmed when he booked a flight to Rome that included a transfer in New York City. When Sitte asked if he might stay at a hotel overnight the New York authorities refused his request, placed him under a heavy police guard, and bundled him onto the first available flight out of the country, which took him not to Italy, but back to Brazil.