After the assassination of Kennedy and the arrest of her husband, Marina was under Secret Service protection until she completed her testimony before the Warren Commission. She made a total of four appearances before the commission. Questions about her reliability as a witness were expressed within the commission, particularly in regard to her claims about an assassination attempt on General Edwin Walker, and her allegation that Lee Oswald had intended to assassinate Richard Nixon. In her testimony, she stated her belief that her husband was guilty, an opinion she reiterated in testimony before the House Select Committee on Assassinations in 1978.