The Taoiseach, Bertie Ahern, tonight revealed he had asked officials to find out the reason IRA bomber Sean Kelly was returned to prison last week.
Mr Ahern said it would be odd if Kelly was arrested without a good reason.
"Either it is a mistake and if it is the authorities look at it, or there must have been substantial evidence that he was not kosher," Mr Ahern said.
"There has to be one or the other. Frankly at this stage I don't know but I intend on finding out."
The Taoiseach told RTE Radio he had asked officials at the Anglo Irish Secretariat to try and find the reasons behind Kelly's return to prison on June 18 last.
Kelly was sent back to jail by Northern Ireland Secretary Peter Hain for breaching the terms of his early release under the Belfast Agreement.
The 33-year-old was originally jailed for bombing a fish shop in 1993 in the Shankill Road area and killing 10.
He was released with other prisoners in July 2000 under the terms of the Belfast Agreement.
Mr Hain said he had ordered the Shankill bomber's rearrest because security intelligence indicated Kelly had become involved again with terrorist activity.
Independent Sentence Review Commissioners will now consider Kelly's case and decide whether to revoke the licence he was given.
Northern Ireland police chief Sir Hugh Orde and the British government have been urged to state clearly the reasons for the arrest.
Nationalist SDLP justice spokesman Alban Maginness said the rearrest, which Sinn Fein president Gerry Adams has described as deplorable, raised serious questions about the prisoner which needed to be addressed.
PA