He was arrested at his father-in- law's house in Strabane, Co Tyrone, where he returned at the weekend. Last year he was spirited out of Northern Ireland by the RUC after being an informer for several years.
Casey has claimed, in a series of articles in the Daily Mirror, that he was involved in 14 IRA murders, six of which were carried out while he worked for the police. He claimed that in two specific cases, killings were carried out even though he had warned his handlers that the individuals were being targeted.
A high-level RUC inquiry has been announced, but at the same time the force's Chief Constable, Sir Hugh Annesley, has already issued assurances that the RUC does not allow any life to be taken in order to protect informers. This will be seen by some observers as pre-empting the investigation.
After meeting Sir Hugh yesterday morning, Sir Patrick Mayhew, Secretary of State for Northern Ireland, told reporters who asked if Casey's allegations were true: 'I have no means of knowing, because I have not got the individual personal knowledge whether it is true or not.
'What I do know absolutely certainly is this - that it would be wholly unlawful, wholly unacceptable, and I am absolutely certain that it would not have been authorised by the Chief Constable or anybody in authority.'
Sir Hugh, in a statement issued a few hours later, confirmed that Casey had been an informer and said his allegations would be fully investigated by a senior detective chief superintendent.
He said the suggestion that the RUC would allow lives to be lost 'has no basis in fact and I totally reject any such suggestion. Of that the people of Northern Ireland can be completely assured.'
Later, Kathleen Finlay, whose husband Ronnie was killed by the IRA, said that she had received a call from the local police commander, who told her that he had been directed by Sir Hugh to say that there was absolutely no truth in Casey's allegations.Reuse content