The Big Operator (1959) starring Mickey Rooney

A film about a corrupt union boss ordering witnesses to be murdered.
Spoiler (just from early in movie):

"The Big Operator (1959) is a crime drama film centered on "Little Joe" Braun, a corrupt union boss in Los Angeles who is under investigation by a U.S. Senate Committee for racketeering. The night before his testimony, Braun orchestrates the murder of the union's treasurer, William Tragg, to eliminate evidence. The hit is carried out by Oscar "The Executioner" Wetzel, who delivers Tragg's incriminating documents to Braun. Two factory workers, Bill Gibson and Fred McAfee, witness this meeting. During his Senate hearing, Braun repeatedly invokes the Fifth Amendment to avoid self-incrimination, vehemently denying any knowledge of Wetzel, a known ex-convict."

I picked this because it's related to the JFK autopsy stuff. All the witnesses were afraid of being killed, every one of them talks about that in their depositions or book interviews.
https://rumble.com/v6qc43i-strange-deaths-penn-jones-talks-to-mark-lane-about-his-investigation.html
https://rumble.com/v6wz2gu-1966-special-report-jfk-assassination-witness-mysterious-deaths.html
https://rumble.com/v6tuiyf-forgive-my-grief-penn-jones-jr-in-1966-dieugenio-intro-2021.html
https://rumble.com/v6q8rwm-coast-to-coast-am-richard-belzer-law-and-order-on-his-book-mass-deaths-of-j.html