From his home in the small village of Donaghcloney, County Down, a few miles south-east of Lurgan, Jackson is alleged to have organised and committed a series of killings, mainly against Catholic civilians, although he was never convicted in connection with any killing and never served any lengthy prison terms. At least 50 killings in Northern Ireland have been attributed to him, according to Stephen Howe (in the ''New Statesman'' magazine) and David McKittrick (in his book ''Lost Lives'').
An article by Paul Foot in ''Private Eye'' suggested that Jackson led one of the teams that bombed Dublin on 17 May 1974, killing 26 people, including two infants. Royal Ulster ConstabularySpecial Patrol Group (SPG) officer John Weir (who was also involved in loyalist killings), also maintained this in an affidavit. The information from Weir's affidavit was published in 2003 in the ''Barron Report'', the findings of an official investigation into the Dublin bombings commissioned by Irish Supreme Court Justice Henry Barron. Journalist Kevin Dowling in the ''Irish Independent'' alleged that Jackson had headed the gang that perpetrated the Miami Showband killings, which left three members of the cabaret band dead and two wounded. Journalist Joe Tiernan and the Pat Finucane Centre also made this allegation and averted to Jackson's involvement in the Dublin bombings. When questioned about the latter, Jackson denied involvement. Findings noted in a report by the Historical Enquiries Team (HET) (released in December 2011) confirmed that Jackson was linked to the Miami Showband attack through his fingerprints, which had been found on the silencer specifically made for the Luger pistol used in the shootings.
Jackson was at one-time a member of the Ulster Defence Regiment (UDR) but was discharged from the regiment for undisclosed reasons. It was stated by Weir, as well as by others including former British Army psychological warfare operative Major Colin Wallace, that Jackson was an RUC Special Branch agent.
