The investigation into Sinn Fein leader Gerry Adams could damage the Northern Ireland peace process, according to the province's Deputy First Minister, Martin McGuinness, who claimed that "dark forces" in the police were behind the arrest.
Mr McGuinness, a former IRA commander, claimed that Mr Adams' arrest over the murder of Jean McConville in 1972 was politically motivated.
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Northern Ireland rocked by Gerry Adams arrest
Northern Irish police questioned Sinn Fein leader Gerry Adams after arresting him under an investigation into 1972 killing.
The Police Service of Northern Ireland refused to rule out making further applications for secret testimonies of 46 former IRA members and paramilitaries held by Boston College in the United States.
Mrs McConville's son, Michael, suggested that another "senior IRA figure" was involved in his mother's death and said that although he knew the identities of her killers, he was too afraid to tell the police.
Mr Adams, the Sinn Fein president, on Thursday faced a second night in custody. He has denied any involvement in the murder and raised concerns over the timing of his arrest. Mrs McConville was dragged from her home in West Belfast in 1972. Her body was not discovered until 2003, when it was found on a beach in Co Louth.
Mr McGuinness said Sinn Fein was being targeted by forces which are "maliciously and vehemently hostile" to the peace process, and described the arrest of Mr Adams as a "deliberate attempt" to influence the outcome of elections in Northern Ireland and the Irish Republic.
He said: "People who could be described as former republicans [are] targeting the Sinn Fein peace strategy and targeting the leader of Sinn Fein, Gerry Adams.
"It has been disappointing to see the efforts of some of those people together in consort with the dark side within policing."
The Daily Telegraph has learnt that Theresa Villiers, the Northern Ireland Secretary, was notified by police that Mr Adams was going to be arrested 24 hours before he was taken into custody.
David Cameron defended the police on Thursday. He said: "There has been absolutely no political interference in this issue."
The investigation of Mr Adams comes after a US judge ruled that police should be given transcripts of IRA veterans discussing the murder of Jean McConville.
Those transcripts were gathered as part of a history project led by Lord Bew, a Northern Irish historian, who spent a year working at Boston College in 1999, where the idea of an oral history of the era known as The Troubles took root.
When he returned to Northern Ireland in 2000, he discussed the idea with the journalist Ed Moloney, and they agreed to set up the Belfast Project, secretively approaching and interviewing "foot soldiers", as Mr Moloney called them, from republican and loyalist paramilitary organisations.
Lord Bew felt that a South African-style truth and reconciliation commission would one day have to be convened in Ulster to cement the peace process, but was aware that it would be many years before the political situation was stable enough to go ahead with it. The Belfast Project aimed to capture evidence ahead of time.
Anthony McIntyre, a former IRA member, was asked to become the project's £25,000 ($45,700)-a-year lead project researcher to conduct the interviews with former republican terrorists. Between 2001 and 2006, hundreds of interviews with dozens of former terrorists were conducted. No one apart from those who were approached knew of the project's existence.
Mr Moloney explained this to a US court: "It is an offence punishable by death for any past or present member to reveal details of the IRA's business to outsiders. Without their interviews being kept under lock and key until death, none of the contributors would or could have conceived of agreeing to participate and neither myself nor Anthony McIntyre would have taken part."
Each interviewee signed an agreement guaranteeing that their interviews would remain in the Burns Library at Boston College until after their death, unless they wished it to be published.
Among those interviewed was Brendan Hughes, a former IRA commander, who died in 2008. His evidence formed the basis for a book by Mr Moloney called Voices From The Grave, and included the allegation that Mr Adams had ordered the killing of the widowed mother of 10 Jean McConville in 1972.
Another interviewee was the Old Bailey bomber Dolours Price, who went public in 2010 with the assertion that she had made a similar allegation to the Belfast Project.
It was at this point that the Police Service of Northern Ireland decided to act.
With the backing of the Home Office, the PSNI contacted the US Department of Justice in March 2011 and issued a subpoena for the production of the Hughes and Price interviews. The Trustees of Boston College fought the case, but the PSNI issued another subpoena that August for "any and all interviews containing information about the abduction and death of Mrs Jean McConville".
Over the course of a two and a half year legal battle, the US courts reviewed 176 transcripts of interviews with 24 people identified as "most likely to have information responsive to the subpoena".
District Judge William Young ruled in January 2012 that 85 interviews should be handed over, prompting Mr Adams's comment that he had "nothing to fear".
Mr Adams, who had not been party to any of the information in the interviews, later added: "This project was flawed and biased from the outset. It was an entirely bogus, shoddy and self-serving effort. It was not a genuine or serious or ethically based history project."
John Kerry, a Massachusetts senator at the time, called on Hillary Clinton to intervene as secretary of state, saying it would be "a tragedy" if this process upset the peace in Ulster.
Owen Paterson, the former Northern Ireland secretary, knew nothing about the PSNI's actions until he read about it in the newspapers. He told RTE, the Irish broadcaster, at the time that while there was "real merit" in gathering oral archives, "we have always said that the rule of law must prevail and that the police have an absolute duty to follow up every possible lead seeking justice for victims and relatives of victims".
Meanwhile, Mr Moloney, Mr McIntyre and Boston College continued to fight in the courts against the PSNI.
Mr Moloney said in one witness statement: "Should our archive be invaded by this action I would be extremely fearful that the consequences for Anthony McIntyre could be more serious."
Their pleas fell on deaf ears. The US government's response in court was: "The respondents made promises they could not keep - that they would conceal evidence of murder and other crimes until the perpetrators were in their graves."
In May last year, five months after Price died in Dublin, the US First Circuit Court of Appeals ruled that segments of 11 interviews with seven interviewees should be handed over, rather than the 85 suggested by the lower court.
In March, the PSNI charged Ivor Bell, from west Belfast, with aiding and abetting the murder of Mrs McConville, which he denies. Five others were arrested and questioned before Mr Adams on Wednesday night.
The Telegraph, UK