The incident happened in a residential building on Via Brioschi.
Among those injured were two sisters, aged seven and 11, who were hospitalized with severe burns, and their father. Their 43-year-old mother died in the explosion, which is thought to have been caused by a leak from a stove in an apartment rented by a couple, who were also among the victims, Ansa reported.
The couple, from the Marche region, were students, both aged 27.
All three victims and the nine injured lived on the third floor of the building, where the gas leak is thought to have occurred.
“The explosion sounded like a bomb – at first I thought it was an attack,” a resident on Via Brioschi was quoted by Ansa as saying.
“Then I realised it was coming from that building."
The man added that the building had recently been restructured.
An investigation has been opened by prosecutor Elio Ramondini.
The incident comes two weeks after fourteen people, seven of them firefighters, were injured in an explosion which completely destroyed a paints factory near Turin.