Provisional IRA Semtex was almost certainly used in the under-car bomb attack which badly injured a prison officer in Belfast, security sources have said.
It is now believed that at least a tonne of the explosive is in the hands of the so-called dissidents, who could have killed the officer and his wife.
She had been standing right beside her husband's VW van only seconds before the explosion outside their home in east Belfast.
The Provos' claim to have decommissioned all their weapons, including an estimated 2.5 tons of Semtex, was met with almost universal approval in 2005 when the IRA claimed it had put all its weaponry, including hundreds of assault rifles, "beyond use".
It is believed the AKS assault rifles used by the killers of David Byrne in the Regency Hotel attack last month also probably came from one of the Provos' un-decommissioned dumps.
Gardai were aware that the decommissioning claims were a convenient political lie to allow Sinn Fein and the DUP form a power-sharing government but they stayed tight-lipped.
The Semtex has turned up on repeated occasions since. Gardai believe two substantial dumps of Semtex, a powerful plastic explosive manufactured by the old Czechoslovakia, were left intact, and that one of these has fallen into the hands of ex-Provos intent on destabilising the political agreement in Northern Ireland.
The prison officer was injured at 7.10am on Friday by the device, which was attached to the front underside of his van. He is still undergoing treatment for serious leg injuries but is expected to survive.
The device used is similar to the under-car bombs (UCBs) used by both republicans and loyalists during the Troubles. A charge of around half a kilo of Semtex is attached by a magnet to the body of the car, usually under the driver's seat.
The PSNI and Prison Service were already on high alert for such an attack and police and prison officers have all been given briefings and repeated warnings in recent weeks.
This is based on intelligence that the dissidents have been planning to mark the centenary celebrations of the Easter Rising by renewing hostilities in Northern Ireland.
This point was reinforced in comments by PSNI Assistant Chief Constable Stephen Martin, who said: "I believe there are people within dissident republican groupings who want to mark this centenary by killing police officers, prison officers and soldiers.
"I am saying that publicly, I am saying it deliberately and I am saying I need the help of the community. That is not inevitable, this does not need to happen, but we need the support of the community."
He said that while the terrorist threat level in Northern Ireland had been at "severe" for a number of years, he would currently describe it as "the upper end of severe".
It is believed that the dissidents gained access to one of two major Provo arms dumps either last year or before that and have control of a major cache of Semtex.
The prison officer who suffered extensive leg injuries on Friday is a former member of the Royal Marines who worked as a trainer in the NI Prison Service's jail and training centre at Hydebank in east Belfast, which houses mainly women and young offenders.