ROME - Investigators in the southern Puglia region said Thursday that they had detained a suspect in a school bombing that killed one student and injured five at a Brindisi school three weeks ago.

After hours of interrogation that started Wednesday, officials said, a 68-year-old fuel vendor from Copertino, about 30 miles north of Brindisi, was arrested on suspicion of attempted multiple homicide with the intent of terrorism. The authorities later identified the man as Giovanni Vantaggiato.

“He admitted to building the explosive device, planting it, and detonating it,’’ said Cataldo Motta, the prosecutor coordinating the investigation, which involves about 100 police and military officers. “He did not want to say why he did it.’’

Formal charges are expected to be filed after a preliminary hearing.

He said the suspect admitted that he had detonated the device during daytime hours instead of at night to make sure that people would be around, a detail that led to the terrorism charge. His objective of creating widespread alarm was achieved, Motta said.

The bomb, hidden near the front gate of a Brindisi vocational school, went off shortly before 8 a.m. on May 19 as students were entering the school.

Because the school was dedicated to Francesca Morvillo, who was killed with her husband, Giovanni Falcone, a prosecutor, by a Mafia bomb 20 years ago, investigators initially investigated links to organized crime. Growing social tensions in Italy and a spate of recent violent acts by radical groups were also considered.

Motta said the suspect had been having “economic problems’’ but could not explain how that might have prompted him to carry out what Motta called a terrorist act.

“He did not explain; he did not give justifications,’’ he said.

The investigations led to Vantaggiato after his car and another, identified by Italian media as belonging to his wife, were recorded by video surveillance cameras in the vicinity of the school several days before the attack and during the early hours of May 19. One car was also present at the moment when the bomb went off, police officials said.

Massimo Bassi, whose 16-year-old daughter, Melissa, was killed in the blast, said he welcomed the arrest.

“Justice has been done,’’ Bassi said at a news conference. The perpetrator “broke my family, but he broke his family as well,’’ Bassi said.

Bassi’s wife, Rita, looked on silently, eyes invisible behind dark glasses.

Motta said the investigation would focus on trying to uncover the motive and to determine whether other people were involved in the crime.

“Now we have a starting point from which to proceed,’’ he said. “The investigation is shaky if we stop here. We must continue to work.’’