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Rome (CNN) -- At least one girl has died and half a dozen are injured following a bomb blast outside a school in the southern Italian city of Brindisi, Italian officials said Saturday.
Officials have given conflicting accounts of the number killed and injured.
Daniela Buccoliero, an official at the Prefecture of Brindisi, a local office of the Interior Ministry, told CNN one 16-year-old girl had died and another six students are injured, two of them seriously.
Brindisi Mayor Mimmo Consales and Fabiano Amati, regional minister for Italy's Civil Protection agency, said there had been two deaths as a result of the blast.
Amati said the scene shortly after the explosion was "dramatic."
"There were school back packs and notebooks everywhere. Many windows of the nearby buildings were broken," he told CNN.
Amati said the police had found three gas cylinders at the site that were detonated with a remote control.
The device was concealed behind a trash can by a wall 50 meters from the entrance of the school, he said.
"It's an attack on all Italians because schools are considered a secure area," Amati said. "It's the first time in our country that a school is under attack."
Consales told CNN one of the girls had died in surgery from the wounds she sustained in the blast.
Another of the injured is in very serious condition, he said.
Italy's Prime Minister Mario Monti said the government was determined to combat crime and unite the country.
He expressed the government's "deep sorrow, dismay and outrage" at what he called a "most grave and heinous crime" in a statement from the United States, where he is attending the G8 summit.
Monti was informed of the blast during the night, and has been in close contact with Italy's president, interior minister and other local authorities since.
He has ordered the country's flag to be displayed at half-staff Saturday and for the next three days, and sent a message of condolence to those directly affected by the blast.
Italians gathered in front of the Pantheon in Rome and at Plebiscito square in Naples late Saturday to protest the violence and to show support for its victims.
Three days of mourning were declared in Brindisi, where schools will be closed through Tuesday.
The explosion occurred early Saturday as students were arriving at the school, which offers vocational training, Italy's ANSA news agency reported.
It is not yet clear why the school was targeted or who carried out the attack.
Police chief Francesco Cirillo told CNN that the motive is under investigation and that no one has claimed responsibility.
Nichi Vendola, governor of the Puglia region where Brindisi is situated, said: "It could be either a mafia or a political terrorism attack. It's too early to say. It's an unprecedented event."
The Francesca Morvillo Falcone school is named after the wife of a prominent anti-mafia judge, which has fueled speculation that the organized crime group might be behind it. It has been 20 years since Falcone was assassinated in Palermo, Sicily, in May 1992.
The school is located near both the tribunal and the city's tax collection agency.
Italy's tax collection agencies, called Equitalia, have been targeted by mail bombs, Molotov cocktails and suicides in front of their offices in recent months, with the incidents occurring in the cities of Rome, Livorno and Bologna, respectively.
Italy's interior ministry said this week that it would start to deploy soldiers outside some government buildings because of the current tensions.
The local anti-racket commission of Mesagne, a town in Brindisi province, issued a warning last week of heightened threats by both organized crime groups and anarchists against government agencies.
An anti-mafia march that scheduled for Sunday will go ahead as scheduled.
Journalists Livia Borghese and Barbie Nadeau and CNN's Joseph Netto contributed to this report.