BELFAST, Northern Ireland  — Northern Ireland's first minister Peter Robinson on Sunday accused Sinn Fein, his partner in government, of a "thuggish attempt to blackmail" police through its criticism of the arrest of its party leader Gerry Adams.

Northern Ireland police on Friday extended Adams' detention, giving detectives until Sunday evening to question him about a 1972 murder, raising the stakes in a case that has rocked the British province.

Nationalist Sinn Fein, which shares power with Robinson's protestant Democratic Unionist Party (DUP), has said the arrest is a deliberate attempt by "dark forces" in the Police Service of Northern Ireland (PSNI) to undermine the peace process.

Deputy first minister Martin McGuinness stopped short of saying Sinn Fein would withdraw its support for the PSNI, a move that would spark a major crisis. But he said it would see if the Adams situation was resolved in a satisfactory manner.

"The publicly conveyed threat to the PSNI 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," Robinson said in a statement.

"The threat now means that ordinary decent citizens will conclude that the PSNI and the PPS (Public Prosecution Service) have succumbed to a crude and overt political threat if Adams is not charged.

"I warn Sinn Fein that they have crossed the line and should immediately cease this destructive behavior."

Adams' arrest over the killing of Jean McConville is 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.

The Sinn Fein leader, who is a member of parliament in the Irish republic, 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.

When he was arrested, Adams said that he was "innocent of any part" in the killing, which he said was "wrong and a grievous injustice to her and her family."

Police, who have been questioning Adams since Wednesday, must decide whether to charge, release him, or seek a further extension in custody.