A five-year-old girl was among crowds of spectators gathered just yards from the car bomb which killed Pc Ronan Kerr before it went off.
The picture of runners taking part in the half-marathon very near the booby-trapped vehicle was released by detectives as they continued to question two men over the murder of the 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 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: "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 on Thursday morning. The 40-year-old, who was stopped in a van close to Omagh, is 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 Northern Ireland today on suspicion of Pc Kerr's murder.
Meanwhile, police in the Republic of Ireland carried out a search in an apartment near to Dublin airport as the investigation developed a cross border dimension. No arrests were made.
Officers north of the border returned to the bomb scene this afternoon in a bid to gather information from the community about the 48-hour window prior to the blast.
As they did so Northern Ireland's Chief Constable, Matt Baggott, warned the killers of Pc Kerr that the hunt to bring them to justice would be relentless.