Marco Toffaloni, 66, believed to be one of those who actually set off the bomb on May 28 1974, was put on trial on charges of murder.
Toffaloni currently lives in Switzerland and is a Swiss citizen.
His defence counsel had asked for a dismissal, arguing that the case had timed out under the statute of limitations.
In another court, a preliminary hearing is underway against another former rightist militant, Roberto Zorzi, who was an adult at the time of the events.
Zorzi is also believed to be one of the material perpetrators.
The government said on March 23 that it would stand as plaintiff in the latest trial into the Piazza della Loggia bombing that killed eight people and wounded 102 more during an anti-fascist rally in 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 for ordering the bombing.
At the time, it was thought to have closed one of the longest-running cases on terrorism during Italy's 'years of lead' of rightist and leftist terror.
Zorzi and Toffaloni are two other Ordine Nuovo-linked former militants.
ANSA
