Irish officials have been asked to find out more about the decision to return to jail Shankill bomber Sean Kelly, Taoiseach Bertie Ahern has said.
IRA bomber Sean Kelly is back in prison
Kelly was convicted of the 1993 bombing that killed 10 people. He was freed early under the Good Friday Agreement.
His early release licence was suspended after security information indicated he had become "re-involved in terrorism".
"Either it's a mistake, or there must have been substantial evidence that he was not kosher," Mr Ahern said.
"It has to be one or the other. At the moment I don't know but I intend to find out."
DUP MP Nigel Dodds said if there was evidence that Kelly was involved again in terrorism, this raised "wider questions" about the IRA's intentions when it was supposed to be debating its future.
Nine life sentences
Kelly was one of two men who planted a bomb in a Shankill Road fish shop. Nine civilians died, as did Kelly's IRA accomplice.
He received a total of nine life sentences but was freed early from prison in July 2000 under the Good Friday Agreement.
Reacting to Kelly's return to prison on Saturday, Sinn Fein President Gerry Adams said it was "unjust" and "reprehensible".
The Chief Constable, Sir Hugh Orde, said Kelly's return to prison followed the terms of the Agreement.
Northern Ireland Secretary Peter Hain said he was "satisfied that Sean Kelly has become re-involved in terrorism and is a danger to others and while he is at liberty, is likely to commit further offences."