Senior Sinn Fein member Gerry Kelly said party leader Gerry Adams is in good spirits inside Antrim Police station, where he has been questioned over the 1972 murder of Jean McConville for the past five days.
Mr Kelly said Mr Adams is concerned about the timing of his arrest, saying it was "political and has been mishandled", and he is worried about the damage the arrest may be doing to the image of policing in Northern Ireland.
He also said he was concerned about the use of so-called "open source material" and he does not know whether Adams will be charged.
Police have until 8pm tonight to release or charge the party leader.
Mr Kelly was being cheered by supporters and jeered by anti-Sinn Fein protesters as he spoke.
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".