The timer in the Omagh bomb would have allowed the bombers to give two hours' warning as easily as the half-hour that was given, the inquests hearing was told yesterday.
Mr Dennis McAuley, a senior scientific officer at the Northern Ireland Forensic Science Agency, described fragments of the bomb's detonating mechanism recovered at the scene and said it was of a type used by dissident republican groups.
He said the improvised timer was in a plastic lunch box. It was probably located in the passenger compartment of the vehicle, with wires leading to the explosive charge in the boot.
Mr McAuley displayed a timer of a similar type, which had been recovered from an explosive device in Belfast after it failed to explode.
The timer was a two-hour electro-mechanical timer, on which the time was set by twisting the knob. But he pointed out that the knob did not provide an accurate means of setting a precise time delay - there was no graduated dial and it was a matter of guesswork.
The coroner, Mr John Leckey, said: "Thirty minutes was the first warning given but really it would be impossible for anyone to set 30 minutes on this timer with precision?" The witness answered: "That's correct."
Mr McAuley also told the hearing he knew of 11 other incidents in 1998 which involved identical timer units. He told the coroner this unit was typical of dissident republican groups: "No type of power unit like that has been used by PIRA [Provisional IRA] that I'm aware of."
He said he had also concluded that the main charge would have been 150-200 kilograms of improvised explosive rather than Semtex. It would have been boosted by an amount of explosive packed into a scaffolding pipe and placed in the middle of the main charge.
Mr McAuley also described other items recovered in a fine-combing of the area, including pieces of the vehicle used, identified from the engine number as a Vauxhall Cavalier. It was maroon coloured and the registration plate MDZ 5211 was found.
The inquests on two boys from Buncrana, Co Donegal, who were among the victims of the bombing, yesterday heard an account of the bus outing with Spanish student friends that brought them to the Co Tyrone town on Saturday, August 15th, 1998, the day of the atrocity.
Shaun McLaughlin (12) and Oran Doherty (8) died along with their friend, James Barker (12) when they caught the full force of the explosion.
The bus driver, Mr Liam McGonagle, of Buncrana, in a statement read to the inquests on Shane and Oran, said that the party on the bus consisted of 30 Spanish children, their three Spanish youth leaders, and 10 local children who were friends of the students.
After visiting the Ulster American Folk Park near Omagh, he drove towards the town at 2.15 p.m. and parked in the Drumragh car-park at 2.40 p.m.
He arranged with the group to return to the bus at 4.15 p.m. and saw them proceed up through the car-park towards Bridge Street. At 2.55 p.m. five local girls and one of the Spanish students returned to the bus. One girl was crying and appeared very upset and he learned from them that there was a bomb scare.
He assumed it was a hoax but shortly afterwards heard a loud explosion. Mr McGonagle said that the remainder of the group did not return to the bus. He took the children who were there to an area he considered safer and told them to remain there while he got a lift to the hospital.
Medical evidence was given of death from multiple injuries. In a personal statement, the State Pathologist for Northern Ireland, Prof Jack Crane, expressed sincere condolences to the families on behalf of himself and Dr Derek Carson, former deputy state pathologist.
Prof Crane said he appreciated that their evidence had been difficult for the families to listen to and he apologised for any distress caused. But the families had shown remarkable courage and fortitude, he commented.
He added: "All of us who worked in the mortuary were affected by this terrible atrocity and I don't think we are ever going to forget the scenes we witnessed." The coroner said it seemed to him "nothing short of miraculous" that more children had not been killed.
A statement on behalf of the Buncrana families said they shared with many of the other families "the frustration that the main perpetrators of this atrocity still take no responsibility for the terrible devastation they have caused. It is a sad reflection on our peoples that there are enough people willing to shelter perpetrators of such an evil act," the families statement said. The hearing continues tomorrow.