Vice President Lyndon B. Johnson assumed the presidency upon Kennedy's death. Marxist and former U.S. Marine Lee Harvey Oswald was arrested for the state crime, but he was shot and killed by Jack Ruby two days later. His personal life has also been the focus of considerable sustained interest, following public revelations in the 1970s of his chronic health ailments and extramarital affairs. Troubled by the long-term dangers of radioactive contamination and nuclear weapons proliferation, Kennedy and Khrushchev agreed to negotiate a nuclear test ban treaty, originally conceived in Adlai Stevenson's 1956 presidential campaign. In their Vienna summit meeting in June 1961, Khrushchev and Kennedy both reached an informal understanding against nuclear testing, but the Soviet Union began testing nuclear weapons that September. In response, the United States conducted tests five days later.