Media playback is unsupported on your device Media caption Kevin McGuigan Sr was murdered at his home at Comber Court in the Short Strand area last week

The BBC understands that police believe members of the Provisional IRA co-operated with a group called Action Against Drugs to kill ex-IRA member Kevin McGuigan last week.

Detectives believe members of both groups joined forces in the murder.

They said they believed it was in retaliation for the murder of former IRA commander Gerard Jock Davison in the Markets area of Belfast in May.

Secretary of State Theresa Villiers has described the situation as "serious".

"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, " she said.

Mr McGuigan Sr was murdered at his home in east Belfast last week.

The PSNI said there is no evidence that Mr McGuigan, a 53-year-old father-of-nine, was involved in the killing of Mr Davison, but republicans and Mr Davison's friends believed he was.

The officer leading the investigation confirmed on Thursday that the police believe Mr McGuigan was killed "by individuals seeking revenge for the murder of Jock Davison".

The police assessment is that those individuals include members of the Provisional IRA as well as a separate, independent group called Action Against Drugs.

Ten years after the IRA announced that it was standing down, the PSNI has now made clear it believes the organisation has not entirely gone away.

How the IRA announced the end of its campaign

On 28 July 2005, the IRA said it had formally ordered an end to the armed campaign from 4pm that day.

Significantly, it was the first time in decades that a republican, former IRA prisoner Seanna Walsh, had been put before a camera to read a statement.

His choice was no mistake: he had been the cellmate of Bobby Sands, the first of the IRA hunger strikers to die.

The statement said: "The leadership of Óglaigh na hÉireann has formally ordered an end to the armed campaign.

"All IRA units have been ordered to dump arms. All volunteers have been instructed to assist the development of purely political and democratic programmes through exclusively peaceful means.

"Volunteers must not engage in any other activities whatsoever."

Two months later came a verification statement from the independent arms decommissioning body that the IRA had in its view put all its weapons beyond use.

When asked by the BBC on Thursday if the assessment that members of the Provisional IRA were involved in the killing means the PSNI believes the organisation still exists, Mr Geddis said: "Members of the Provisional IRA were involved in this killing. I'm not going to speculate as to what the Provisional IRA looks like but, yes, that is my information."

But the officer's assessment suggests the police believe those involved may have been operating as a group of individuals, rather than following orders from what used to be called the IRA's Army Council.

"I have no information at this stage to say whether that was sanctioned at a command level or not and I'm not prepared to speculate on that," Mr Geddis said.

"We are at the early stages of this investigation and we come back to one or two people who have been arrested and it is my belief that members of the Provisional IRA were involved. I have no information to say any more in relation to at what level."

Whether the killed was "sanctioned" or not, for the first time since the IRA said it was standing down, the police have now said clearly they believe it still exists.