He then gave zip ties to the three adults, ordering them to tie each child’s wrists together as he drove. As they went back the rows of seats, the adults left the zip ties progressively looser, another prosecutor, Francesco Greco, said.

Police in two carabinieri cars, tipped off by the phone call, forced the bus to stop on the Paullese provincial highway, but Mr. Sy rammed the cars again and again. While some officers tried to speak to him, others from a third military police car forced the bus’s side door open and broke the windows at the back to allow the passengers to flee.

At that point, Mr. Sy apparently ignited a lighter he was holding, officers said, and the bus caught fire as the hostages were escaping.

Italian television stations showed images of the charred skeleton of the bus, completely destroyed.

Asked why the adults did not try to overpower the driver, Warrant Officer Palmieri said that he could not speak for them, but that “the threat was very real.”

Mr. Sy was born in Senegal in 1972, investigators said, and became an Italian citizen in 2004. His wife, from whom he is separated, is Italian, and the couple have two children.

Mr. Sy had been working for the Autoguidovie bus company for 15 years, a company official, Corrado Bianchessi, said. He “never gave any particular signal” to indicate what he had planned, Mr. Bianchessi told the Italian channel of Sky News. “We are amazed,” he said.

Deputy Prime Minister Matteo Salvini, who is also Italy’s interior minister and a major opponent of illegal immigration, posted a tweet in which he said that Mr. Sy had a police record for driving under the influence and sexual assault.