Detectives questioning Sinn Fein leader Gerry Adams over the murder of Belfast mother-of-10 Jean McConville are to ask a judge for more time to interview him, Martin McGuinness has said.
With the initial 48-hour deadline looming for officers to either charge or release Mr Adams after his arrest on Wednesday night, the PSNI is to apply for an extension, the Deputy First Minister said.
Adams, 65, who vehemently denies allegations levelled by former republican colleagues that he ordered Mrs McConville's murder and secret burial, now faces a third night in custody at Antrim police station.
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 the station by prior arrangement with detectives.
The Deputy First Minister 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."
'Review' PSNI support
Mr McGuinness said Sinn Fein would "reflect" and "review" its support for policing in the region if Mr Adams is charged.
But he urged republicans to remain calm if and until that happened.
"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," he said.
"Obviously in the context of the scenario we find ourselves in at the minute we will have to, on an ongoing 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."
Sinn Fein's decision to sign up to support the police in 2007 is viewed as a major milestone and prompted the return to devolved rule in the region, with the republican party and the Democratic Unionists entering government together.
Withdrawal would be a huge blow to the peace process.
Dolores Kelly, the SDLP's spokesperson on policing, said Mr McGuinness’ threat to withdraw support was "incredibly ill-judged".
"The SDLP fought and won the Patten reforms to ensure that policing was not subject to political interference," she said.
"Sinn Féin belatedly accepted these reforms so it is incredibly ill-judged that Martin McGuinness would now make such comments which are clearly in contravention of both the word and the spirit of the Patten reforms. It is very far from where, in the assessment of the SDLP, either nationalists or unionists are.
She added: "For Sinn Féin to threaten to withdraw support from the PSNI because an investigation is not going the way they want it completely unacceptable... For anyone, or any party to try to unpick the progress we have made is just wrong. Sinn Féin must realise that these threats are very damaging."
"No dark forces"
Meanwhile, Northern Ireland's justice minister has said he can see no sign of dark forces operating within the police service.
Martin McGuinness on Thursday criticised some elements within the Police Service of Northern Ireland for detaining Gerry Adams three weeks before the European elections.
Stormont Justice Minister David Ford said: "If there are dark forces within policing, I can see no sign of dark forces.
"I see a police force with very high levels of confidence, higher than the Garda Siochana (Irish police) or many forces in Great Britain, and I see a police force carrying out its duties properly and appropriately, following up evidential opportunities where they present themselves and operating in conjunction with the community across a range of issues."