Nationalist politicians have expressed serious doubts that loyalists alone were responsible for the murder of Mr Billy Stobie.
Sinn FΘin's policing spokesman, Mr Gerry Kelly, said that while a loyalist may have pulled the trigger, elements of the security services could have been involved.
"Many people will be convinced this killing smells of RUC Special Branch and British military intelligence getting rid of an embarrassment and a potential problem down the road.
"These suspicions will be compounded by the timing of the shooting on the day the spotlight is firmly on the Special Branch and their role in the Omagh bombing."
SDLP Assembly member, Mr Alban Maginness, said: "There are some very unusual, mysterious and sinister aspects to this murder. The UDA evidently carried it out but were there others involved too? A lot of questions have to be answered."
The Minister for Foreign Affairs, Mr Cowen, said: "This murder was abhorrent in itself. It may also have been an attempt to stifle the search for the truth into the circumstances surrounding the murder of Pat Finucane. It must not, and will not, be allowed to halt or hinder that search."
The human rights organisation, the Committee on the Administration of Justice, said the killing removed a crucial witnesses who would have given evidence to a public inquiry on the murder of Mr Pat Finucane.
There would be no public confidence in an unsupervised police investigation into Mr Stobie's death, the CAJ said. The human rights organisation, British-Irish Rights Watch, said the Police Service of Northern Ireland should not be allowed to investigate the killing because Mr Stobie had made accusations about the police in the North.
The group said it had "learned with shock but sadly no great surprise" of the murder. Officers from an outside force had to be brought in to investigate it, the group said.
The Finucane family expressed shock at the Stobie murder. In a statement, they said:
"The family did not want him murdered nor did they even want him prosecuted. All they wanted was the truth. There have been too many murders and too many grieving relatives.
"If a public inquiry had been established into Pat's murder instead of the Stevens police investigation, Billy Stobie could have been granted anonymity and his identity unknown and he would probably still be alive today."
Mr John White, the former chairman of the UDA's political wing which was recently disbanded, said there could be no justification for murder but he was not surprised by the killing.
Mr Stobie knew death was the penalty for becoming an informer when he joined the UDA, he said.
"Going on television and making a broadcast about his involvement would have created a lot of anger.
"He also supported an inquiry into the killing of Pat Finucane. Most people within loyalism see this as republicanism taking advantage of the death of Pat Finucane to undermine the credibility of the RUC and Northern Ireland as an entity in itself."
Mr Nigel Dodds, a DUP MP, said it was "a horrific murder" which served no cause. Mr Fred Cobain, an Ulster Unionist Assembly member who sits on the Policing Board, described Mr Stobie's killing as "shocking and brutal".
He said: "Irrespective of Mr Stobie's past, nobody has the right to take his life. I am sure the police will undertake a thorough investigation into what has happened."
The Alliance leader, Mr David Ford, and the North's Security Minister, Ms Jane Kennedy, also condemned the killing .