If no one takes responsibility, it may not be terrorism at all. Maybe it is just random violence? Sadism? Terrorism has a political objective.
While on the subject of words. Those inured were not heroes, they are victims. A hero performs heroic deeds, many of the first responders were heroes. Getting blown up does not make one a hero.
One more. The perpetrators were not necessarily cowards. It takes a certain level of guts and/or bravery to assemble a bomb and deliver it to a crowd of people. There is both an immediate and a long term chance that you will be killed or imprisoned, it is not a cowardly act.
We are taught that bravery is good and cowardliness is bad. But good people can be cowardly and bad people can be brave. Quite often these are judgments based on your political views, and devolve into jingoism to be parroted by politicians wishing to capitalize on the tragedy of others.