He wrote books such as "The High Priests of War", in which he criticized the neoconservatives in the Bush administration, and "Final Judgment", where he claimed that Israel's Mossad, the CIA and organized crime was responsible for the assassination of U.S. President John F. Kennedy. He had been criticized by many organizations, including the Anti-Defamation League, the Simon Wiesenthal Center, and the Middle East Media Research Institute, which have described Piper as a promoter of antisemitic conspiracy theories and Holocaust denial.