Lt William Pitzer was in the room filming the autopsy of John Kennedy. That was his job at the Bethesda Naval Hospital.

In October of 1966 he was found dead in the film lab with a gunshot would and the gun by his side. His death was ruled a suicide and the film was gone. Naval intelligence people told his wife that it was best not to talk to anyone about this.

Special Forces Lt Daniel Marvin approached her sometime later and told her that Lt Pitzer died at the hands of someone in organized crime. They were the ones who did the CIA dirty work domestically. The CIA initially wanted Marvin to carry out the hit, but military personnel only do jobs out of the country. 

It seems that powerful people were concerned that he would take copies of his work to film studios and show the truth of what happened.

Without Smoking Gun: Was The Death Of Lt. Cmdr. William B. Pitzer Part of the JFK Assassination Cover-up Conspiracy?
by Kent Heiner

This shocking account of intrigue, lies, and governmental complicity provides dramatic evidence that suggests a larger conspiracy behind JFK's assassination. Three years after Kennedy's assassination, Lieutenant Commander William Bruce Pitzer, who was reputed to have in his possession documents and film that refuted the conclusions of JFK's official autopsy, was found dead in his office at the National Naval Medical Center in Bethesda, Maryland. In 1995, a retired special forces captain claimed that a representative of the CIA recruited him to assassinate Pitzer. This, as well as the mysterious circumstances of Pitzer's death and the official and nonofficial investigations that followed, are outlined. These revelations of a possible conspiracy within a conspiracy raise larger questions of the measures taken to suppress the truth and the potential dangers of a government that operates outside the law.