Murdered loyalist Malcolm McKeown is suspected of ordering a £50,000 hit on a republican prisoner currently serving a sentence in Maghaberry Prison.
McKeown (54) was shot six times in the head and body at a petrol station at Waringstown, Co Down, last year.
Police later said the victim was gunned down following a "long-standing" criminal gang feud involving drugs.
Details of the bounty emerged when Luke O’Neill, from Lurgan, took legal action after being refused entry to the republican wing at Maghaberry Prison by authorities.
O’Neill is currently serving a six year sentence after pleading guilty last year to two counts of the attempted possession of explosives, namely an "improvised anti-armour explosive device''.
After his arrest in 2016 O’Neill was held in the general prison population in Maghaberry Prison’s Quoile House.
He was said to be so concerned for his safety that he refused to leave his cell and did not integrate with other inmates.
Since his conviction he has been moved to Roe House, which holds republican prisoners.
A High Court judge yesterday turned down an application for a judicial review into the decision to house O’Neill in Quoile House.
The legal move also sought a declaration that the decision was unlawful.
A judge ruled that “because of the change in circumstances that no useful purpose can be served by the continuation of these proceedings”.
The threat against O’Neill was the subject matter of a Prison Service Security Information (SIR) Report.
A “gist”, which outlines the contents of a sensitive report, was later disclosed to his legal team.
The document reveals that a prisoner reported that a sum of money from an unidentified source had been offered to him and other prisoners “’to do in’ the applicant”.
The sum of money referred to in the report was £50,000.
The SIR was later assessed and given an ‘E4’ grading meaning the source was untested and the reliability of the information could not be judged.
O’Neill’s solicitor Gavin Booth, of Phoenix Law, said his client believes Malcolm McKeown was the source of the threat.
He added that his client was “forced to barricade himself in his cell 24 hours a day which he attached to Malcolm McKeown placing that threat on his life”.
McKeown was a member of a well-known loyalist family.
The murdered loyalist is a brother of loyalist supergrass and sectarian killer Clifford McKeown, who is currently serving a life sentence for the murder of Catholic taxi driver Michael McGoldrick in July 1996.
Another brother Trevor McKeown was convicted of shooting dead Catholic teenager Bernadette Martin as she slept in her Protestant boyfriend’s family home in the village of Aghalee, Co Antrim in July 1997.