Sinn Fein leader Gerry Adams is being questioned for up to 17 hours a day by detectives investigating the 1972 murder of Jean McConville, a source close to him has told the BBC.
The source said a "significant development" would need to occur for Mr Adams to be charged.
Police have until 8pm to charge or release Mr Adams, who has spent four nights in custody.
Adams attacks reasons for his detention and denies McConville murder
It is claimed IRA volunteers described what happened the night Jean McConville was murdered to Boston College's "Belfast Project".