Italy was in shock on Saturday after a 16-year-old girl died and five other teens were gravely injured in a bomb blast at a school, as investigators cast doubt on possible involvement by the mafia. There were scenes of chaos and carnage when the powerful blast went off at the entrance of the building just as students were arriving for morning classes at the vocational high school in Brindisi in mafia-heavy southern Italy. "I was opening the window and the blast wave hit me. I saw kids on the ground. All blackened. Their books on fire. It was terrifying," an employee at the prosecutor's office next to the school told the Repubblica daily. Another teenage student who witnessed the blast told local television: "I had just gone into the bar in front of the school. I saw everything falling." A middle-aged woman, still under shock, said: "I saw a girl lying on the ground and another one who got up and started shouting: 'Melissa! Melissa!'" The victim was identified as Melissa Bassi, who later died of her injuries in hospital, Fabiano Amati, a local representative of the civil protection agency, told news channel Sky TG24. The explosive was composed of three gas canisters with a timer device hidden in a container next to a wall just outside the school, Italian media reported. Debris could be seen scattered over a wide area at the scene of the blast. Hospital official Paola Ciannamea said that one of the injured girls, who is also 16, was in a "very serious condition" with injuries to her chest. The four other victims being treated are all suffering from extensive burns and five more have been discharged from hospital with lighter injuries. The blast went off at around 7:45 am (0545 GMT). Most Italian schoolchildrn and students attend classes on Saturday. Students at the school, which teaches social work, tourism and fashion, were due to hold a fashion show later in the day. The bomb caused shockwaves across Italy, with Prime Minister Mario Monti ordering flags flown at half-mast for three days. The government called it "an extremely grave and atrocious criminal act." The president of neighbouring France, Francois Hollande, expressed his country's "deep solidarity" with Italy in the face of this "odious attack".