Northern Ireland police released Sinn Fein leader Gerry Adams after four days of questioning over the 1972 abduction and murder of Jean McConville. (Reuters)

Police in Northern Ireland on Sunday freed Sinn Fein leader Gerry Adams without charge four days after he was detained for questioning in a four-decade-old murder case that has reawakened old divisions and challenged a still-fragile peace.

Adams had been held in custody since Wednesday evening, answering questions related to the 1972 killing of Jean McConville, a mother of 10 who was suspected by the Irish Republican Army of working as an informant for the British. The killing, at the height of the Troubles that roiled Northern Ireland for decades, has never been solved.

But police, working with new evidence gleaned from an oral history project conducted by Boston College and featuring interviews with IRA veterans, suspect that Adams ordered the killing in his role as the IRA’s Belfast commander.

Although he was freed Sunday, the 65-year-old Adams was not exonerated. Instead, police handed a file of potential evidence to British prosecutors. The decision means it is still possible that Adams could be charged, though most analysts deem that unlikely.

Even without charges, the case has badly shaken a power-sharing arrangement in Northern Ireland that has been a cornerstone of peace since the U.S.-brokered Good Friday accords of 1998.

Before Adams was released, pro-unionist First Minister Peter Robinson accused Sinn Fein, his coalition partner in the Northern Ireland Assembly, of “a despicable, thuggish attempt to blackmail” police into freeing Adams. The accusation came after Sinn Fein suggested that it could withdraw its support for the police force if charges against Adams went forward.

“I warn Sinn Fein that they have crossed the line and should immediately cease this destructive behavior,” said Robinson, who leads the Democratic Unionist Party.

Robinson’s deputy, Sinn Fein’s Martin McGuinness, had earlier blamed Adams’s arrest on “a dark side within policing here in the north of Ireland.” Leaders of the Irish nationalist party claimed that the arrest was politically motivated, coming as it did just weeks before Sinn Fein is to compete in local and European elections.

In a Belfast news conference Sunday evening, Adams repeated that charge, saying his arrest amounted to “the old guard using the old methods.” He also denied involvement in McConville’s death.

But Adams was more conciliatory than McGuinness had been, saying that Sinn Fein would continue to support the police force and was “totally and absolutely committed to the peace process.”

Elsewhere in Belfast, one of McConville’s children, Michael, said the family would continue to push for justice.

The decision to free Adams, who is popular among Catholics in both Northern Ireland and the Republic of Ireland, is likely to anger Protestants who feel he has repeatedly cheated justice. A group of them protested Sunday evening outside the gates of the police detention center in Antrim, where Adams had been held.

Adams has long denied ever being part of the outlawed IRA, a militant Catholic group that fought for decades to end British rule in Northern Ireland and to unify with the Irish republic. Unlike in Ireland, Catholics are a minority in Northern Ireland, which remains part of the United Kingdom.

The IRA has since renounced violence and disbanded, although dissident republican groups continue to carry out occasional, low-level attacks. About 3,600 people died in the decades of violence known collectively as the Troubles.

The IRA in 1999 admitted to killing McConville and said the group thought she was a spy, though a subsequent investigation found no evidence to support that claim.