In the early 1990s, Fetzer started promoting John F. Kennedy assassination conspiracy theories, later 9/11 conspiracy theories, Holocaust denial, conspiracy theories regarding the 2002 death of Senator Paul Wellstone and more recently 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.
Fetzer was born in Pasadena, California, on December 6, 1940, to a father who worked as an accountant in a welfare office in Los Angeles County, and grew up in a neighboring city, Altadena.
After his parents' divorce, Fetzer moved to La Habra Heights, California, with his brother, mother, and stepfather. His mother died when he was 11, and he went to live with his father and stepmother.
Following his graduation from South Pasadena High School, Fetzer studied philosophy at Princeton University and graduated magna cum laude in 1962, while his undergraduate thesis, done under Carl G Hempel, won The Dickinson Prize. He then joined the United States Marine Corps, and was second lieutenant in an artillery unit. In the early 1960s he was stationed at Okinawa, Japan. During military service in the 1960s, Fetzer married, and divorced four years later, after having a son. He remarried in the 1970s while teaching at the University of Kentucky.
In 1966, soon after promotion to captain, he resigned to enter graduate school. Having attained a master's degree from Indiana University, he studied at Columbia University for a year, then returned to Indiana University and in 1970 gained a PhD in history of science and philosophy of science.
He became an assistant professor at the University of Kentucky in 1970, and received the University of Kentucky Student Government's first Distinguished Teaching Award in 1973. He was denied tenure at Kentucky in 1977, and spent the next ten years in visiting positions at the University of Virginia, University of Cincinnati, University of North Carolina at Chapel Hill, and University of South Florida. After ten years without a tenure-track position, in 1987 he was hired as a tenured full professor at the University of Minnesota Duluth. In 1996 Fetzer received a Distinguished McKnight University Professorship from the University of Minnesota, a title that recipients retain until they retire from the University. He retired in 2006 and is now an emeritus professor.
In the late 1970s, Fetzer received a National Science Foundation fellowship, and contributed a chapter to a book on Hans Reichenbach. In 1990, Fetzer received the Medal of the University of Helsinki. He assisted theorists in computer science, and joined debate over proper types of inference in computing. In the late 1990s, Fetzer was called to organize a symposium on philosophy of mind, and authored textbooks on cognitive science and artificial intelligence. He is an expert on philosopher Carl G. Hempel.
Fetzer published over 100 articles and 20 books on philosophy of science and philosophy of cognitive science, especially of artificial intelligence and computer science. He founded the international journal Minds and Machines, which for 11 years he edited, and founded the academic library Studies in Cognitive Systems, of which he was series editor. He founded the Society for Machines & Mentality. Near and after retirement, Fetzer remained a contributor to as well as cited or republished in philosophy of science and cognitive science volumes and encyclopedias.
Fetzer was formerly a staff writer for the website Veterans Today, but left along with several other writers after a dispute with the editorial board.
Claiming an interest in alleged government conspiracies that dates to the 1963 assassination of US President John F. Kennedy, Fetzer became "a familiar and controversial figure in the JFK research community". With Don "Four Arrows" Jacobs, Fetzer claimed the 2002 airplane crash that killed US Senator Paul Wellstone was an assassination. Fetzer alleged the 9/11 attacks were treason and called for military overthrow of President George W. Bush. In 2005, with Steven E. Jones, Fetzer co-founded Scholars for 9/11 Truth. Fetzer has asserted that the World Trade Center buildings collapsed by controlled demolitions or by high-tech weaponry, although his speculations have drawn further critics. An article by Fetzer published by Press TV and Veterans Today (a site, according to Oliver Kamm, which "promotes conspiracy theories") titled (by the latter) "Did Mossad death squads slaughter American children at Sandy Hook?" was described in January 2013 by Kamm in The Jewish Chronicle as "monstrous, calumnious, demented bilge" that "violates all bounds of decency". Jovian Byford criticized Fetzer's speculations that Jews or Israel were involved in a conspiracy to commit the 9/11 attacks as "a contemporary variant of the old, antisemitic conspiracist canard about the disloyalty of Jews and their usurpation of power in the name of communal interests and the accumulation of wealth." Fetzer has written: "My research on the Holocaust narrative suggests that it is not only untrue but probably false and not remotely scientifically sustainable." In 2015, Fetzer published a book titled Nobody Died at Sandy Hook: It Was a FEMA Drill to Promote Gun Control.
In 2013, officials of the University of Minnesota said that "Fetzer has the right to express his views, but he also has the responsibility to make clear he’s not speaking for the university." He is retired and no longer employed by the university.