In the early 1990s, Fetzer began to promote John F. Kennedy assassination conspiracy theories, later 9/11 conspiracy theories, Holocaust denial, conspiracy theories regarding the 2002 death of Senator Paul Wellstone and Sandy Hook Elementary School shooting conspiracy theories. He cofounded Scholars for 9/11 Truth in 2005, and claims that the United States government, Israeli government and Israeli Mossad are involved in these and other conspiracies. Fetzer's allegations and speculations have drawn strong criticism. In October 2019, a Wisconsin court ordered Fetzer to pay the father of a Sandy Hook victim $450,000 in a defamation case.