The bomb attack which killed a teenaged girl and wounded 10 others in the southern Italian town of Brindisi was probably done by an individual with no links to the mafia, a senior official said on Sunday.
The attack on the Francesca Morvillo Falcone school, a vocational training institute offering courses in fashion, tourism and social services, has horrified Italy.
Thousands have taken to the streets in demonstrations of sympathy for the school and the family of Melissa Bassi, the teenager who died in the explosion.
"It seems to be the work of a single person," Marco Dinapoli, the Brindisi chief prosecutor who is leading the investigation, told reporters on Sunday.
"The most probable hypothesis is that it was an isolated act," he said, but refused to give details about the person believed to have carried out the attack.
"At the moment, we don't understand what the motivation for this massacre might be," Mr. Dinapoli said, adding that no claim of responsibility had been received.
Early suspicions pointed at organized crime, largely because the school was named after the wife of murdered anti-mafia judge Giovanni Falcone and the attack took place days before the 20th anniversary of the couple's death in a bomb attack in Sicily.
As the investigation has proceeded, the idea that either the Sicilian mafia or the local version known as the United Sacred Crown, might mount such a high profile attack has appeared increasingly unlikely.
"It seems improbable, not entirely to be excluded but improbable, that it is connected with mafia networks," he said, adding that repeat attacks on other schools appeared unlikely.
Mr. Dinapoli said investigators had acquired "significant" video evidence which suggested that one man had set off the device, which exploded as pupils at the school were getting off a bus and arriving for the start of lessons on Saturday morning.
He declined to describe the video evidence but said it showed a single individual activating a remote control of some kind to detonate a rudimentary bomb made up of three gas canisters hidden in a container outside the school gates.
In Brindisi, a quiet port town on Italy's Adriatic coast, there was a palpable feeling of shock, with businesses throughout the town carrying signs of mourning and anti-mafia messages still visible from a demonstration on Saturday.
Another girl, Veronica Capodieci, has been transferred to a hospital in the larger city of Lecce. She is still in serious condition, but the hospital reported on Sunday that she was stable, had regained consciousness, and was alert.
Whoever was behind it, the deadly attack on a school was a potent symbol for an aging country grappling with economic decline and struggling to regain confidence in its future.
"What a terrible thing for that poor girl's family," said 63-year-old Donatella Rosario, sitting on a bench in the early morning sunshine. "You send your child to school and it's supposed to be safe. How can you live in a country where they kill our children?"
On Sunday, the pope added his commiserations to messages of sympathy from leaders including French President François Hollande, saying he was praying for "Melissa, the innocent victim of brutal violence and her family".
Italy's collective nerves were already on edge after a series of attacks on public institutions, including the main tax and debt collection agency which prompted Interior Minister Anna Maria Cancellieri to step up security at a series of key sites.
As the economic crisis has hit hard, most visibly through a series of high-profile suicides of struggling small businessmen, there have been fears of a return to the kind of violence seen in the 1970s and 1980s.
Italy coped with a severe terrorism outbreak in those two decades — known as the "years of lead." In the worst attack, blamed on right-wing terrorists, 85 people were killed in a bomb blast at the Bologna train station in 1980. A Mafia terror campaign targeted churches and public buildings in Rome and Milan.
Saturday's bombing follows a spate of recent attacks against Italian officials and government or public buildings by a group of anarchists, including the shooting and wounding of an official from a nuclear engineering firm, which is part of a state-controlled company. An anti-nuclear anarchist group that previously had targeted Italy's tax collection agency claimed responsibility for the shooting.
With a report from The Associated Press