The stay-behind part was just cover for action. It was used to perpetrate right-wing terrorist actions against the civilian population, disguised as leftist terrorism to justify repressive government programs.
http://en.wikipedia.org/wiki/Bologna_Central_StationTerrorism
On August 2, 1980 at 10.25 am, an improvised explosive device (IED) made with 20 kilograms of a TNT mixture, detonated inside Bologna's main train station. The IED was contained in a suitcase, which was placed near the wall inside a waiting lobby. The explosion killed 85 people and injured more than 200. The wing of the station in which the bomb detonated has been reconstructed but, as a testimony to the victims of the attack, the original pavement was maintained as well as a deep crack in the main wall. Moreover, the station main clock is forever stopped at the exact time of the explosion. The attack is also known in Italy as the Strage di Bologna, the Bologna massacre.
The Italian government immediately accused the Italian based militant group Red Brigades for the attack, however this was denied by the group, and no one ever claimed responsibility. Over the years, official investigation has determined that the attack was probably carried out by a small Neo-Fascist group. Many conspiracy theories regarding this event have been entertained, including a link between this act of terrorism and the Aerolinee Itavia Flight 870 disaster.
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This was probably the most spectacular but there were many others.