The bombing occurred during an anti-fascist rally in the northern city of Brescia.
In 2015, a court of appeal in Milan issued a final life sentence to rightist militant Ordine Nuovo (New Order) members Carlo Maria Maggi and Maurizio Tramonte.
They were found guilty of ordering the bombing, closing one of the longest-running cases of terrorism during Italy's 'years of lead', a period of rightist and leftist terror.
Currently two other Ordine Nuovo-linked former militants, Roberto Zorzi and Marco Toffaloni, are on trial for actually carrying out the bombing on May 28, 1974.
Initial reports claimed the government had not decided whether it would stand as civil plaintiff in the case.
The trial is set for a preliminary hearing later Thursday.
ANSA
