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Adams' arrest over the killing of Jean McConville was among the most significant in Northern Ireland since a 1998 peace deal ended decades of tit-for-tat killings between Irish Catholic nationalists and mostly Protestant pro-British loyalists.
Northern Ireland Deputy First Minister Martin McGuinness, a Sinn Fein member and close Adams ally, said the decision by the Police Service of Northern Ireland (PSNI) to seek an extension confirmed his view that the arrest was politically motivated.
"There is a cabal in the PSNI that have an agenda, a negative and destructive agenda to both the peace process and to Sinn Fein," McGuinness told a news conference in Belfast. Northern Ireland's justice minister denied the accusation.
"I believe Gerry Adams will be totally and absolutely exonerated and I believe that Gerry Adams will continue to lead this party," said McGuinness, a former Irish Republican Army (IRA) commander who only last month met Queen Elizabeth at a Windsor Castle reception in a sign of normalised ties between Britain and Ireland since the peace pact.
The decision extends Adams' detention at a police station about 25km northwest of Belfast until 7pm on Sunday (local time). Adams, who led the IRA's political wing in the 1980s and 1990s, must then be freed or charged unless police can convince a judge to agree to an additional extension.
After the extension was announced, a single elderly man appeared outside the police station holding an Irish tricolour. He was heckled by passers-by.
Reviled by many in Britain as the face of militant Irish nationalism during the IRA guerrilla campaign against British rule of Northern Ireland, Adams, 65, reinvented himself as a Northern Ireland peacemaker and then a leading opposition parliamentarian in the Irish Republic.
But he has been dogged throughout his career by accusations from former IRA fighters that he was involved in its campaign of killings, a charge he has repeatedly denied.
He offered to speak to police about McConville's killing in late March after tapes that apparently accuse him of participation were released by researchers in the United States.
Adams has always denied membership of the IRA and said on Wednesday, when he was arrested, that he was "innocent of any part" in the killing, which he said was "wrong and a grievous injustice to her and her family".
McGuinness said it might be difficult to contain anger among Irish nationalists about Adams' detention.
"We believe that the anger and resentment out there among the community is something we as Irish republicans have to manage. We are trying to handle this situation in a very calm way," he said.
The investigation of former militants on both sides of the conflict have stirred protests in the province in recent years, but there were no signs of trouble in Belfast city centre early on Friday evening.
Some motorists passing the police station where Adams was being held sounded their horns and shouted "Free Gerry!"
A mural was being painted on the mainly Catholic Falls Road with a picture of Adams beside the words "Peacemaker, Leader, Visionary".
The creation of a neutral police force was one of the foundations of Northern Ireland's peace deal and any perceptions of bias could be hugely damaging.
But Northern Ireland Justice Minister David Ford, who is also the leader of the non-sectarian Alliance Party, rejected McGuinness' accusation that police were unfairly targeting Sinn Fein over pro-British loyalists and British servicemen.
"In the four years I've been minister for justice I've seen no evidence of the police playing politics but I've certainly seen much evidence from many politicians seeking to interfere in the policing, prosecution and judicial process," Ford said outside the police station where Adams was being held.
McConville, who was dragged away screaming from some of her 10 children, was one of 15 people living in strongly Republican, Catholic areas who were spirited away by the IRA and dumped in unmarked graves.
Her remains were found only in 2003 by a man walking on a beach over the border in County Louth, a jurisdiction Adams now represents in Ireland's parliament. The IRA accused McConville - a Protestant married to a Catholic - of being an informer for the British, an allegation her family has always denied.
The investigation into her killing was revived by the release of a series of taped interviews given by former guerrillas from the Northern Ireland conflict for a research project at Boston College in the United States.
U.S. Secretary of State John Kerry said during a visit to South Sudan on Friday that he was unfamiliar with details of the Adams case but that the legal process in Northern Ireland needed to be allowed to "work its way".
The United States brokered the 1998 Northern Ireland peace accord and has a large number of citizens of Irish heritage.