He started promoting Holocaust denial ideas and spreading conspiracy theories in the 1990s. In 2005 Fetzer co-founded the Scholars for 9/11 Truth movement. He contributes to the antisemitic American website Veterans Today and regularly appears as a commentator for the Iranian channel Press TV.
Fetzer ascribes to the theory that Mossad and the United States government are involved in the assassination of John F. Kennedy, the attacks of September 11, 2001 and the Sandy Hook school massacre in Newtown, Connecticut.
The titles of his books—several of which are self-published—speak for themselves: Assassination Science: Experts Speak Out on the Death of JFK (1998), Murder in Dealey Plaza: What We Know Now that We Didn't Know Then (2000), The Great Zapruder Film Hoax: Deceit and Deception in the Death of JFK (2003), The 9/11 Conspiracy: The Scamming of America (2007), Nobody Died At Sandy Hook: It was a FEMA Drill to Promote Gun Control (2015).
In July 2015 Fetzer wrote a blog post commenting on the book by British Holocaust denier Nicholas Kollerstrom, declaring that "the narrative about the Holocaust” is “political propaganda”. According to Fetzer “the Holocaust story appears to be false and cannot be sustained”.
Fetzer was ordered in October 2019 by a Wisconsin court to pay $450,000 to Leonard Pozner, father of Noah Posner, a 6-year-old victim of the Sandy Hook shooting. He is the co-author of Nobody Died at Sandy Hook with Mike Palacek. Fetzer claims that the December 14, 2012 massacre never happened and that it was an event staged by the federal government in order to legislate for gun control. He accused Leonard Pozner of fabricating copies of his son Noah's death certificate.