Sinn Fein leader Gerry Adams remains in police custody as detectives question him over his alleged role in the Irish Republican Army killing of a Belfast mother of 10 in 1972.
Under British anti-terrorist law, Adams must be charged or released by Friday night, unless a judge approves an extension.
Senior politicians in Adams' Irish nationalist party have accused those behind Wednesday's arrest of the 65-year-old of pursuing an anti-Sinn Fein agenda.
Adams denies any role in the outlawed IRA. Former members who spoke on tape to a Boston College-commissioned research project have linked him to the abduction, slaying and secret burial of Jean McConville, a 38-year-old widow whom the IRA branded a British spy. An official 2006 investigation dismissed this claim.