Italy was in shock on Saturday after an unexplained bombing at a school killed a 16-year-old girl and left five other teens gravely injured, sparking emotional protests across the country.
There were scenes of chaos and carnage when the powerful blast went off at 7:45 am (0545 GMT) near the entrance to the building as students were arriving for classes at the vocational high school in the southern city of Brindisi.
"There was huge black smoke, debris all over the place, piercing screams," Corradino De Paolis, a passer-by who rushed to the scene, told AFP.
Debris including charred notebooks at the site of the blast was spread over a wide area as investigators combed the area in search of evidence.
Victim Melissa Bassi's friend, Veronica Capodieci, also 16, was in a "very serious condition" with injuries to her chest, hospital doctors said.
The four other victims being treated were all suffering from extensive burns and five more had been discharged from hospital with lighter injuries.
No one has claimed responsibility for the blast and Interior Minister Anna Maria Cancellieri said investigators were looking into "numerous hypotheses."
The explosive was composed of three gas canisters with a timer device hidden in a container next to a wall just outside the school, which teaches social work, tourism and fashion and is named in honour of a victim of the mafia.
The region where the attack took place is a hub of the Sacra Corona Unita (United Sacred Crown), a local mafia that has been under pressure from investigators in recent years and whose influence is seen as being on the wane.
The group, which is heavily involved in drug and arms smuggling through the Balkans as well as human trafficking, is believed to be behind a separate bomb attack in the region earlier this month against an anti-mafia campaigner.
Observers pointed to the fact that school is named after Francesca Morvillo, the wife of famous anti-mafia judge Giovanni Falcone, who was assassinated with her husband and three bodyguards by a mafia bomb 20 years ago on Wednesday.
But officials cautioned that it was unlikely the Sacra Corona Unita would target civilians in its own territory and said the device used was not sophisticated enough for an organisation that has easy access to explosives.
"Local mafia organisations want social consensus. This act would go against their interests because it would alienate any sympathy," said local chief prosecutor Cataldo Motta, adding: "Too many coincidences could be just that."
Thousands of young people led protests in several cities and many said they believed the attack could be linked to the mafia or political militancy at a time in which Italy is struggling through a painful economic crisis.
"Cowards!" read a banner held up at a protest in the heart of Rome.
"Every time someone tries to change something in Italy, there's always something dramatic, an attack to terrorise people," said one man in the crowd, Enrico Fontana from the anti-mafia group Libera.
Along with several other participants, he likened the school bombing to a wave of attacks carried out by far-right and far-left militants in the 1970s and 1980s in a period known as the "Years of Lead".
Thousands also took to the streets of Brindisi to voice their anger.
"We have to react!" one student told the crowd in Victory Square.
"Crime in Brindisi is nothing new but it's been hushed up. We can't take it any more!" the student shouted to cheers from the crowd. "We don't need a massacre to realise there's a problem. We are strong, we must not be afraid!"
Speaking on the sidelines of a G8 summit at Camp David in the United States preoccupied with the economic crisis, Italian Prime Minister Mario Monti condemned the bombing as a "tragic", "criminal" and "unprecedented".
He ordered flags to be flown at half-mast for three days across Italy.
Monti called for "maximum cohesion of all political and social forces to prevent the return to our country of subversive temptations."
French President Francois Hollande, who was also at the summit, expressed his country's "deep solidarity" with Italy in the face of this "odious attack".
The Vatican called it a "an absolutely horrific and vile act."