The girl was shot at around 7am on Wednesday morning and died in hospital 24 hours later, after several cardiac arrests. Her presumed attacker has also been found dead.
The attack took place in a central street of Ischitella in Foggia in the southern Puglia region as the 15-year-old was walking to her bus stop.
Dozens of police officers and a helicopter were involved in a search for the suspected attacker on Wednesday afternoon, and an arrest warrant was issued against the ex-partner of the girl's mother, aged 37.
But the suspect was found dead in nearby countryside 12 hours after the crime, having apparently killed himself using the same weapon he turned on the girl, regional paper Foggia Today reported, citing initial police reports.
The victim's mother had reported her ex-partner to police twice, the most recent occasion just two weeks ago, after she claimed he had threatened her daughter. The girl was in the care of her grandparents and social services were in contact with the family.
Writing in a Facebook post, the girl's mother wrote: "I had warned that something would happen, but no-one believed me."
She had written just the previous day that it had been exactly a month since leaving the relationship, which she described as "a prison where I no longer knew if I could see the sun, if I could breathe, if I was myself".
Italy's interior minister, Marco Minniti, said in August that extra units of special investigators from Italy's top security agencies would be relocated to Foggia following four daylight shootings linked to the local mafia.
Meanwhile, the President of the Chamber of Deputies, Laura Boldrini, on Tuesday called for more robust laws to protect women who are victims of gendered violence following a series of reports of violent assaults across the country in recent weeks.