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Rome (CNN) -- A bombing at a school in Italy was likely "the isolated action of a single man" and not tied to the mafia, an official said Sunday.
Surveillance video shows a suspect that authorities believe is an adult, said Marco Dinapoli, attorney general in Brindisi province.
The possibility that an accomplice was involved has not been ruled out, he told reporters.
Earlier Sunday, Dinapoli told CNN that police were questioning two men in connection with Saturday's blast, which killed a 16-year-old girl and injured six other people outside a school in the southern Italian city of Brindisi. One of the injured is in very serious condition, Brindisi Mayor Mimmo Consales said.
The explosion caused a chaotic scene at the school, said Fabiano Amati, regional minister for Italy's civil protection agency.
"There were school backpacks and notebooks everywhere. Many windows of the nearby buildings were broken," he said.
Flowers with messages for the student who died were left at the school's entrance Sunday, Italy's ANSA news agency reported.
Police found three gas cylinders at the site that were detonated with a remote control, authorities said.
The device "wasn't very sophisticated but done by someone who knew electronics," Dinapoli said Sunday. Someone detonated it from a close distance, he said.
Police were still investigating the motive behind the attack, Dinapoli said.
"It could be a fool or someone angry with the world or with some ideological goal, a terrorist," he said.
At demonstrations throughout the country, Italians expressed solidarity with the victims and condemned the violence.
"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."
Italian 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 was attending the G8 summit.
Monti has ordered the country's flag to be displayed at half-staff through Tuesday, 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.
The explosion occurred early Saturday as students were arriving at the school, which offers vocational training, Italy's ANSA news agency reported.
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 last 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 scheduled for Sunday is expected to proceed as scheduled.
Journalists Barbie Nadeau and Livia Borghese and CNN's Joseph Netto contributed to this report.