The suspension system fitted to a school bus involved in a fatal crash had an "inherent" design defect, a court has heard.
The school bus went out of control on a bog road just outside Clara, Co Offaly, on 4 April 2006 after the rear-drive axle came off.
Schoolboy Michael White, 15, died as a result of "catastrophic injuries" suffered during the crash.
It is alleged that a missing bolt and fractures in the rear suspension springs resulted in the axle separating from the bus.
David O'Reilly, acting on behalf of vehicle testing firm O'Reilly Commercials Ltd of Ballinalack, Mullingar, Co Westmeath, has pleaded not guilty to four charges relating to failing to note or verify defects between 5 and 6 August 2005.
On day 11 of the trial at Dublin Circuit Criminal Court, Garda Adrian Tucker told Diarmuid McGuinness SC, defending, that after the crash he sent out a notice to PSV inspectors about the potential defects in similar buses.
In the letter Garda Tucker said these buses should be checked as a matter of urgency.
He wrote that during the crash it appeared that the rear-drive axle had become fully detached as a result of fatigue fractures that had developed in the main support members of the Air Ride suspension system.
He said that the original conversion in 1991 of the buses to incorporate the new suspension system was carried out to a very high standard but that the potential for the axle to detach was always present.
He told counsel: "There certainly seemed to be an inherent defect in the design. There was an inherent potential for failure in this design".
He said that he would certainly have concerns as a tester if he came across a vehicle with a modified suspension system.
Garda Tucker told Caroline Biggs SC, prosecuting, that such a modification could be a cause of potential danger, saying: "Any modification to a suspension system would raise serious concern".
He said that regular maintenance and inspection of such a system is "critical".
He agreed with Mr McGuinness that the 1991 fitting of the Air Ride suspension system by Scottish firm Alexander to the fleet of Mercedes buses was no "back street modification".
He told counsel that he was not aware if the UK authorities were notified of the modification.
Garda Tucker agreed that it would be hard not to notice that a bolt was missing from the suspension system during an authorised test.
He told Ms Biggs that the testimony of school children who said they heard rattling and banging from the bus was consistent with metal surfaces slapping together as a result of a missing bolt.