That is the hour in which students come: a 16 year old girl died and her friend is dying.
The attack has not been claimed, but because the school is named after the wife of Giovanni Falcone, the Sicilian judge who was killed by the Mafia in 1992, also died in the attack on the husband, this particular, might suggest that the origins of this attack is Mafia.
Would not be the first time that the Mafia does the attacks directed at Italian citizens, the latter are the 1993, but it does not appear that Cosa Nostra has never used the gas cylinders, it must be said, however, that in Brindisi, as in the whole of Puglia, the dominant criminal organization, is the "Sacra Corona Unita" ...
UPDATE: here is 2 frames, taken by a camera that show a person - perhaps the alleged bomber - that he is hiding behind a plant.
Source: IL Manifesto