The picture, showing the girl waving to runners taking part in the half-marathon right beside the booby-trapped vehicle, was released by detectives as they continued to question two men over the murder of the young Catholic officer.
Police believe the high explosive device had already been planted when the image was taken in Omagh, Co Tyrone, on Saturday afternoon. Two hours later it detonated with devastating consequences when 25-year-old PC Kerr got into his black Ford Mondeo to go to work.
The officer in charge of the investigation, Detective Superintendent Raymond Murray, said anyone could have been caught up in the blast.
“This device had the potential to kill or maim anyone who happened to be in the area when Ronan got into his car — whether that was him, two men taking part in a fun run or an innocent five-year-old girl watching the race,” he said.
“We already know it was murderous. Now we have evidence that it was potentially indiscriminate.”
Officers arrested a second man in connection with the murder yesterday morning.
The 40-year-old, who was stopped in a van close to Omagh, was also being questioned about a substantial arms and explosives cache uncovered by the murder investigation team in nearby Coalisland on Tuesday night.
A 26-year-old arrested in Dunbartonshire in Scotland on Wednesday over the weapons haul was re-arrested in police custody in the North yesterday morning on suspicion of PC Kerr’s murder.
Meanwhile, gardaí carried out a search in an apartment near to Dublin Airport as the investigation developed a cross-border dimension. No arrests were made.
PSNI officers returned to the bomb scene yesterday in a bid to gather information from the community about the 48-hour window prior to the blast.