Officers from the national police force broke windows at the back of the bus to reach the passengers and got all of them out without serious injuries before flames destroyed the vehicle, authorities said.

The driver was apprehended and treated for burns. Prosecutors described him as a 47-year-old Italian citizen of Senegalese origin and said he told authorities he wanted to vindicate Europe-bound migrants who have died in the Mediterranean Sea but did not plan to hurt anyone.

However, prosecutors said the suspect, identified as Ousseynou Sy, made preparations that showed his actions were premediated, such as buying a canister of gasoline and restraints on Tuesday.

He also sent a video to friends in Italy and Senegal indicating plans for a bold action and with the message, "Africa, Rise up," they said.

Sy was being investigated on suspicion of kidnapping, intention to commit a massacre, arson and resisting law enforcement, with terrorism as an aggravating circumstance since the event caused panic.  Prosecutors said they have found no evidence of Islamic radicalization or ties to extremists, saying it appeared the bus driver acted alone.

Chief prosecutor Francesco Greco praised the Carabinieri for moving swiftly to block the bus and remove the children.

"They carried out an operation that we see in films with special agents," Greco said.

"Thank goodness, because by then the intent to massacre had ignited, and the man was starting to set fire to the bus, as he did," he said.

Milan's provincial Carabinieri commander, Luca De Marchis, told broadcaster Sky TG24 that the bus was ferrying two middle-school classes between school and a nearby gym in Cremona province, some 40 kilometers (25 miles) from Milan.

The driver threatened the passengers, telling them "no one would survive today" as he commandeered the bus, De Marchis said.

Italian news agency ANSA quoted one of the students as saying the driver everyone's phones and ordered the chaperones to bind the students' hands with cable ties, threatening to spill gas and set the bus ablaze. ANSA said the chaperones only loosely bound several students' hands with the zip ties, not everyone's.

One of the middle school students described his terror in an interview with La Repubblica TV, his face obscured due to his age. His name was not given.

"We were all very afraid because the driver had emptied the gas canister onto the floor (of the bus.) He tied us up and took all the telephones so we could not call the police," the student said.

"One of the telephones, belonging to a classmate, fell to the ground, so I pulled off the handcuffs, hurting myself a bit, and went and picked it up. We called the Carabinieri and the police."

Authorities said that an adult called an emergency operator, while one of the students called a parent, and they alerted authorities, who set up roadblocks. The bus was intercepted on the outskirts of Milan by three Carabinieri vehicles, which were able to force it into the guardrail, De Marchis said.

"While two officers kept the driver busy — he took a lighter and threatened to set fire to the vehicle with a gasoline canister on board — the others forced open the back door, breaking two windows," De Marchis said.  While the evacuation was still underway, the driver started the blaze.

De Marchis credited the officers' "swiftness and courage," for getting out all the children and their teachers "with no tragic consequences."

Some of the passengers were treated at a hospital, mostly for cuts and scratches related to the evacuation, he said.

Prosecutors confirmed that Sy, who became an Italian citizen in 2004, had been convicted in 2007 and 2011 of drunken driving and sexual molestation of a minor. Sky TG24 said that the driver had worked for the bus company for 15 years without any employment-related issues.

"Investigators must clarify how the transport company permitted such a delinquent ... to drive a bus, especially one carrying children," said Riccardo De Corato, a Milan provincial official for security.

Video showed firefighters dousing the bus that had been completely gutted by flames, leaving only the charred metal frame.