Sinn Fein leader Gerry Adams was released from police custody yesterday amid a war of words over his detention.
A senior Northern Ireland police officer said authorities would send an evidence file for potential charges to British prosecutors.
Police on Wednesday arrested Adams, 65, to question him over allegations he was the Irish Republican Army's Belfast commander in 1972 and ordered the killing of a Belfast mother of 10.
Adams denies the allegations. His detention has rocked the fragile government in Belfast.
First Minister Peter Robinson attacked Adams' Sinn Fein party for "despicable" remarks indicating that it would review its involvement with policing in the province after the arrest, which Sinn Fein says was politically motivated.
Sinn Fein's support for the police was a key part of the 1998 Good Friday agreement that ended decades of Protestant-Catholic violence in Northern Ireland and led to the creation of a power-sharing government in Belfast.
But tensions have risen since Wednesday's arrest of Adams, a key figure in the peace process, over the murder of Jean McConville.
A source close to Adams, who has strongly denied any involvement in the murder or membership of the IRA, told the BBC that the political leader had been questioned "for 17 hours a day".
"The publicly conveyed threat to the [Police Service of Northern Ireland] delivered by the highest levels of Sinn Fein that they will reassess their attitude to policing if Gerry Adams is charged is a despicable, thuggish attempt to blackmail the PSNI," said Robinson.
"I warn Sinn Fein that they have crossed the line and should immediately cease this destructive behaviour," he said, suggesting the future of Northern Ireland's government was at stake.
Robinson's Protestant conservative Democratic Unionist Party, which wants Northern Ireland to stay in the United Kingdom, shares power with Catholic republican Sinn Fein, which was the political wing of the nowdefunct IRA and wants Northern Ireland to join the Republic of Ireland.
Northern Ireland's devolved government was suspended on several occasions between 1998 and 2007 due to political disagreements, but has operated uninterrupted since 2007.
During a rally on Saturday by hundreds of Sinn Fein supporters at the unveiling of a new painted mural of Adams, Deputy First Minister Martin McGuinness said the arrest was "political policing."
McGuinness, a former IRA commander, said there were still "dark forces" in the police who were opposed to the peace process, and added that the party would review its relationship with police in Northern Ireland.
McConville's children watched as she was dragged screaming from their Belfast home in 1972 after the IRA accused her of being an informer. Her remains were found buried on a beach in 2003 and tests found she had been shot in the back of the head.
Additional reporting by Associated Press