On the 2nd picture he has sunglasses, but not on the 1st or 3rd...? Completely different guy ?
Also where did the dude go to use two bulky RC car transmitters to set off the bombs? Can't be far away. Portapotties? Look for one with two RC car transmitters dropped in them? I would imagine that would help with DNA analysis too, having 1000s of peoples samples in there.
So the vigilantes need to find pix of the portapotties, which I suspect were within a a hundred feet of the blasts. In fact their location combined with the predicted blast size probably defined where the blasts happened as the bombers would want to be just outta range of the blast while still having a great RC car signal.
TLDR is the witch hunt should be searching for two guys with bulky backpacks and one guy with a weirdly shaped pack who left the portapotties with a much smaller backpack right after the two booms.
EDIT: after finding the higher-resolution photos, it's almost certainly the same 2 guys in each pic. On the other hand, the descriptors label him/them as moving, when my impression is that they're just standing there. Worthy of study, sure, but pretty weak sauce by itself.
In the first picture, they're standing next to the lamp post near the tree (you can see the branches on the left). Also note the no stopping sign facing inwards.
In the last picture, they've moved further away from the finish line to the next lamp post - you can see the first lamp post and tree in the background. Also note the no stopping sign on the second lamp post is facing the road.
Less obviously, you can see the Slovakia flag, third to the left of the lamp post in the first picture. Harder to make out in the second picture, but it's clearly a white/blue/red tricolor.
I'm guessing they're not significant because while they first drew attention for both carrying heavy-looking bags (a black backpack and a blue duffel), that may be a subconscious bias towards the fact that there were two explosions. On the other hand, the two explosions were separated by a city block; so unless one or both of them were spotted a similar distance away from the start point, the fact of the two big bags is probably just coincidence.
So many people were taking photos and video that by the time the investigation is done, I imagine there'll be a patchy but contiguous animation of people's movements through the area, at which point it'll be far easier to spot dichotomies in behavior between race viewers and the criminal(s) that carried out the bombing.