In February 1950, Cvetic surfaced for the first time and was summoned to testify before HUAC. During this testimony he was accompanied by several Communist Party members from Pittsburgh who he had been working with, and they did not find out he was an undercover FBI agent until he began testifying against them, despite having accompanied him to the hearing. During his testimony in February 1950 and in subsequent sessions throughout the rest of the year, Cvetic named several hundred people as members of the CPUSA, and provided information regarding the internal workings of the Pittsburgh branch of CPUSA as well as various allegedly communist-controlled organizations. However, while Cvetic was considered a reliable witness when testifying in front of friendly audiences, hostile cross-examinations often left him at a loss for words, unable to remember key details and often refusing to answer questions on the grounds that he did not wish to give away FBI secrets to the communists. While testifying at the trials of CPUSA leader Steve Nelson for sedition in 1951 and 1952, Cvetic was questioned on the stand by Nelson who, acting as his own counsel, repeatedly referred to Cvetic as "a fingerman" and "professional patriot." Cvetic continued to appear at various government hearings until 1955.