The existence of an IRA structure that can be "activated at will" is causing serious concern among rank-and-file police.
Officers are on heightened alert following the security assessment that current IRA members were involved in the murder of republican Kevin McGuigan.
The Police Federation for Northern Ireland, the body that represents officers, warned that the PSNI's official assessment of IRA activities was a cause of "great concern". "It is worrying," said federation chairman Mark Lindsay. "Our members view developments with great concern."
On Thursday the PSNI said that Mr McGuigan was murdered as part of a "joint enterprise" between the IRA and the Action Against Drugs group.
Yesterday Secretary of State Theresa Villiers described the situation as serious. She tweeted: "This is a serious situation. It is vital that police are able to get on and pursue their lines of enquiry."
As the fallout continued, a 60-year-old man was arrested for questioning in relation to Mr McGuigan's murder. He remained in police custody last night. So far 10 people have been arrested, including Shankill bomber Sean Kelly, who was later released unconditionally. Another six men and a woman who were detained for questioning were also subsequently released without charge.
One man, Patrick Gerard Fitzpatrick (53), has been charged with possession of a pistol.
Mr McGuigan, a former IRA prisoner, was gunned down outside his home in the Short Strand area of east Belfast. It is strongly believed he was killed in revenge for the murder of Gerard Davison.
Last night Sinn Fein president Gerry Adams broke his silence as pressure mounted on Sinn Fein over the murder. He said the claim of IRA involvement was a bid "to undermine Sinn Fein's mandate; the rights and entitlements of our electorate, and the peace process".
He added: "The killing of Kevin McGuigan was wrong and those who were involved in it are criminals who do not represent republicanism. The 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 criminal ends. My thoughts are with the McGuigan family. Anyone with any information should bring it to the PSNI.
"There has been speculation and media spin about whether the IRA was involved in the killing. The IRA was not involved."
An urgent meeting of the Northern Ireland Policing Board has been called for by UUP board member Ross Hussey. Mr Hussey said that following the PSNI's assessment "logic would suggest there is still an IRA Army Council and GHQ staff". "The confirmation that PIRA members were involved in the killing proves that the organisation still exists and still has the ability to act with lethal effect," claimed Mr Hussey.
He added: "The fact that several individuals appear to have been involved in the murder, the fact that it appears to have been methodically planned and the fact that the PSNI have confirmed that the Provisional IRA collaborated with other terrorist organisations all confirm that the organisation never fully disbanded and never fully surrendered its weapons." However, Sinn Fein's Gerry Kelly said the killing was the responsibility of "a criminal gang" involved in extortion, intimidation and murder in nationalist communities.
"What is required now is mature leadership from all of our political parties in standing together against criminal gangs trying to bring us back to the past," added the Sinn Fein MLA.