Detectives have been granted a further 48 hours to question Sinn Fein leader Gerry Adams over the IRA murder of Belfast mother-of-ten Jean McConville.
With the initial 48-hour deadline looming for officers to either charge or release Mr Adams after his arrest on Wednesday night, the PSNI successfully applied to a Belfast judge for an extension.
Mr Adams, 65, who vehemently denies allegations levelled by former republican colleagues that he ordered Mrs McConville's murder and secret burial in 1972, now faces a third night in custody in Antrim police station.
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A PSNI spokesman said: "Detectives from Serious Crime Branch investigating the abduction and murder of Jean McConville in 1972 have been granted an extra 48 hours to interview a 65-year-old man who was arrested in connection with the investigation on Wednesday 30th April."
The Sinn Fein president's party colleague and Stormont deputy first minister Martin McGuinness denounced the police application as more evidence of what he has branded "political policing".
He claimed a "cabal" within the PSNI was behind the arrest, with the intent of damaging the peace process and inflicting political scars on Sinn Fein in the month of an election.
Mr McGuinness indicated that Sinn Fein would review its support for policing in Northern Ireland if Mr Adams is charged by detectives investigating the murder of Mrs McConville.
The deputy first minister said he and colleagues would not be making a "knee-jerk" decision, but suggested they would "reflect" on their endorsement of the PSNI if such a situation came to pass.
Mrs McConville was dragged screaming from her children in the Divis flats in west Belfast by a gang of up to 12 men and women.
She was interrogated, shot in the back of the head and then secretly buried - becoming one of the "Disappeared" victims of the Troubles. Her body was not found until 2003 on a beach in Co Louth, 50 miles from her home.
Mr Adams, a former MP for West Belfast and now an elected representative for Co Louth in the Irish Dail, voluntarily presented himself for interview at Antrim PSNI station by prior arrangement with detectives.
Mr McGuinness raised the spectre of what would be a huge blow to the peace process in the region as he expressed his anger at Mr Adams's on-going detention.
Asked if Sinn Fein would withdraw support for policing if Mr Adams is ultimately charged, Mr McGuinness said: "We are very thoughtful and we are very reflective but I think if such a scenario does develop then we will sit down and we will reflect on what will be an even more serious situation than the one we face today."
Mr McGuinness also said: "Yesterday I said that the timing of the arrest of Gerry Adams was politically-motivated.
"Today's decision by the PSNI to seek an extension confirms me in my view."
Sinn Fein's decision to sign up to support the police in 2007 is viewed as a major milestone in the peace process and prompted the return to devolved rule at Stormont, with the republican party and the Democratic Unionists entering government together.
Again touching on Sinn Fein's attitude to the police if Mr Adams was charged, Mr McGuinness told a media conference at the party's headquarters in west Belfast: "Obviously in the context of the scenario we find ourselves in at the minute we will have to, on an on-going basis, monitor this situation where our party leader is being detained and I think you can draw your own conclusions.
"Depending on what happens this scenario will either be resolved in a satisfactory way, in which case we will continue to press on, continue to support the reformers within policing who have made, I think, such a massive contribution to the change of the policing arrangements that we have enjoyed in the course of recent times or the situation will not work out in the way we believe that it should.
"If it doesn't, we will have to review that situation and we will have to review that situation in the context of continuing with our very positive and constructive role within what is a vitally important peace process."
Last night the Prime Minister urged the leaders of Northern Ireland's power-sharing government to cooperate after the arrest prompted sharp divisions between them.
David Cameron spoke to First Minister Peter Robinson and Mr McGuinness, who head the devolved administration at Stormont, amid heightened republican anger at the timing of the Mr Adams' detention just before an election.
Northern Ireland Secretary Theresa Villiers said today: "The Prime Minister does appreciate the sensitivity of a case of this nature. He, like me, is anxious to ensure that the devolved institutions continue to work well and that the Northern Ireland political leaders continue to work with each other on the big challenges that we face."
Mr McGuinness has blamed the "dark side" of policing conspiring with enemies of the peace process for the arrest of his friend and party leader.
He acknowledged that Mrs McConville was the victim of a terrible wrong done by the IRA but said Wednesday's action was a deliberate attempt to influence the outcome of European elections due in three weeks' time.
DUP leader Mr Robinson said it would have been political policing if the PSNI had decided not to investigate Mr Adams because of the pending poll.
It is the latest dispute which has bedevilled the Stormont regime.
While uniting to condemn violence by dissident republicans opposed to the peace process, many key decisions remain in limbo because of differences between Sinn Fein and the DUP.
At the end of last year, the five main Stormont parties failed to agree on dealing with the legacy of 30 years of violence which has left thousands of victims, controversial Orange Order and republican parades and the display of the British flag on public buildings.
The Northern Ireland Secretary recognised the difficulties created by the arrest for relations between the parties.
She said: "That is why I am urging everyone to respond in a measured way.
"Whatever the outcome, there are still problems to be fixed."
Ms Villiers defended the police decision to arrest Mr Adams now, saying detectives should not be asked to take on board the electoral calendar as well as their other duties.
She said: "The Prime Minister is clear that this is not a political decision. This is a matter for the police and prosecution authorities.
"There has been, and there will be, no political interference in prosecutions in this case.
"I don't think we can expect the police in their decisions to take account of the electoral calendar. If they were doing that, that probably would be a political decision.
"They have to take steps to follow the evidence and take steps based on their timetable."
She said the force was doing a good job for Northern Ireland.
"I believe that the PSNI take their duties in terms of impartiality, balance and respect for human rights extremely seriously.
"They are not a politicised police force, they are properly and highly scrutinised by the Policing Board and Police Ombudsman."
Irish Tanaiste (deputy prime minister) Eamon Gilmore said politicians should not receive special treatment when it comes to police investigations.
"I don't think anybody, Sinn Fein or anybody else, should seek to have a different rule applied to somebody because they are a politician," said the Labour Party leader.
"If the police service want to interview somebody in connection with a murder investigation, they are perfectly entitled to do that.
"If that person happens to be a politician or the leader of a political party, so be it. There isn't a separate rule."