Stephen Rakes, 59, of Quincy was killed in July. His body was found in a wooded area of the Boston suburb of Lincoln, just a day after he learned he wouldn't be called as a witness against Bulger.
Rakes openly despised Bulger and blamed him for seizing control of his South Boston liquor store to use as headquarters for Boston's Irish mob in 1984.
Authorities said Rakes' death is not linked to Bulger's case.
Rakes' business associate, William Camuti, 69, of Sudbury, is charged with attempted murder and other crimes for allegedly poisoning Rakes' iced coffee. Camuti has pleaded not guilty.
Prosecutors plan to file additional charges against him based on the new findings.
Bulger is scheduled to be sentenced next month for 11 murders and dozens of other gangland crimes.