Sinn Fein president Gerry Adams has insisted that the IRA was not involved in a Belfast murder.
The assessment by commanders in the Police Service of Northern Ireland (PSNI) that members of the Provisional IRA (PIRA) were involved in the killing of father-of-nine Kevin McGuigan last week has raised the prospect of a political bid to oust Sinn Fein from Stormont's power-sharing government.
However Mr Adams said: "There has been a lot of speculation and media spin about whether the IRA was involved in the killing of Kevin McGuigan. The IRA was not involved."
He said that in July 2005 the IRA left the stage. Its leadership ordered an end to the armed campaign and instructed its volunteers to take part only in "purely political and democratic programmes" and no "other activities whatsoever".
He added: "All of this was done as part of a genuine initiative to build a just and lasting peace and in support of the full implementation of the Good Friday Agreement.
"Those who threaten to take action against Sinn Fein in the Assembly and Executive have no basis whatsoever for this. Sinn Fein's mandate and the rights and entitlements of our electorate deserve exactly the same respect and protection as anyone else's."
Earlier the Police Federation for Northern Ireland (PFNI), which represents the rank and file, described the PSNI's assessment as "worrying".
PFNI chairman Mark Lindsay said: "This was a deliberate and measured assessment by the PSNI, conscious that every syllable would be placed under the microscope and weighed and analysed.
"It is a very worrying development if a command structure can be activated at will. Our members view developments with great and justifiable concern."
A 60-year-old man arrested in connection with the shooting has been released without charge.
The arrest was the eighth made by detectives so far.
Detectives investigating the murder have so far charged one man with possessing a weapon with intent to endanger life.
The accused - Patrick John Fitzpatrick, 53, from Lagmore Dale in west Belfast - was remanded in custody at Lisburn Magistrates' Court on Thursday. The other six held in relation to the shooting were subsequently released.
Stormont First Minister Peter Robinson on Thursday indicated he will hold talks with other Executive parties about a potential Sinn Fein exclusion from the mandatory five-party coalition administration.
The IRA has been on ceasefire since 1997 and decommissioned its weapons in 2005.
On Thursday the PSNI said it suspected current members of the PIRA of involvement in the murder of former IRA man Mr McGuigan.
Detectives have said they are not in a position to assess whether the killing was ordered by a command structure within the outlawed and supposedly defunct organisation.
Mr McGuigan was gunned down in east Belfast in a suspected feud between former IRA members.
He was shot dead in front of his wife Dolores outside their home in Comber Court in the republican Short Strand on August 12.
He was suspected by some in the republican movement of involvement in the murder of former IRA leader Gerard "Jock" Davison in the nearby Markets area of Belfast three months ago.
There has been widespread speculation that his killing was a revenge attack by Mr Davison's one-time republican associates.
PSNI Detective Superintendent Kevin Geddes said: "A major line of inquiry for this investigation is that members of the Provisional IRA were involved in this murder."
Mr Geddes said police were investigating whether a criminal group calling itself Action Against Drugs was behind the killing.
He noted that the group had issued a public statement earlier this month threatening to "execute" anyone it believed was involved in the Davison murder.
The detective said while Action Against Drugs was made up of "criminals, violent dissident republicans and former members of PIRA" he said current members of the PIRA are also suspected of involvement in the murder. But he insisted Action Against Drugs was a "separate" organisation from the PIRA.
Mr Adams said: "The killing of Kevin McGuigan was wrong and those who were involved in it are criminals who do not represent republicanism. The so-called group Action Against Drugs is a criminal gang. It is a mix of criminals and former republicans who have engaged in intimidation and violence in pursuit of their criminal ends.
"My thoughts are with the McGuigan family. Anyone with any information on this brutal murder should bring it forward to the PSNI.
"So too should anyone with information on the killing of Jock Davison.
"The events surrounding the murders of Jock Davison and Kevin McGuigan and their investigation are matters for the PSNI."
Northern Ireland Secretary Theresa Villiers has described the situation as "serious".
She tweeted: "This is a serious situation. It is vital that the police are able to get on and pursue their lines of enquiry. I urge anyone with information to contact the police in confidence."