The annonimity that crowds give the bombers, make them very difficult to track down. Moreover the people in the proximity of the bomb would be dead and cannot give any leads to the police. Only with sheer luck can the perpeterators be identified and nabbed before they leave the country. Which brings us to my point that the intention of the bombers should indicate their identity rather than investigations. There are many posts here that call for "completion of the investigation", but it is an nearly impossible task. The policy of guilty until proven innocent should be used.
