The investigations focused on the extra-parliamentary left and Potere Operaio's militants were investigated. The defendants, Achille Lollo, Manlio Grillo and Marino Clavo, were first released due to lack of evidence. They were later sentenced, although for less serious crimes (arson and manslaughter) than that of massacre originally contested, to 18 years of imprisonment.

The first acquittal allowed the defendants to flee abroad, eventually obtaining that the sentence imposed was declared time-barred. Lollo took refuge in Brazil before the final verdict of the Cassation and his sentence was declared extinguished on 12 October 2003.  In 2005, in an interview, he admitted to have carried out, with others, a demonstrative attack with a homemade bomb that did not explode, aimed to Mario Mattei, but always maintained that he had not set fire to the house with petrol.  Six years later he returned to Italy and explained that the action was carried out by a group of six people in all (including himself) and that the action was intended to be exclusively demonstrative.

We also have a testimony of a classmate of Stefano(we didn't write the name because se wants to stay anonime):

“I was born and raised and still live in the Primavalle district. This was considered a "hard" neighborhood especially in my childhood and adolescence years or the 70s. It is known that throughout Italy, especially in Rome, these were the years of terrorism when the political rivalries between right and left were so strong that it was really difficult and dangerous to leave the house. Growing up here in Primavalle meant being at the center of turmoil and political clashes almost daily. I was in elementary school when the Primavalle fire occurred, where Stefano and Virgilio Mettei died. Stefano was my classmate and a neighbor of mine... I was little but I remember everything: the smoke... the cries of the Mattei family... a daughter who threw herself off the balcony… the people of the neighborhood who told the other children to throw themselves down, that they would have caught them… but Stefano and Virgilio didn't and they died in that atrocious way amidst the cries of their parents and the people of the neighborhood… I remember the siren of the fire brigade, the ambulance and the police…

The burnt door of that house stayed that way for a long time and every time I passed by it I shivered. Not to mention the return to class where flowers had been placed on Stefano's desk in his memory. I remember that we had a very strict teacher, who used a wooden wand to hit us on the hands if we spoke, as was the custom in those years. He never mentioned the fact of the stake, as if it were something we couldn't talk about... perhaps out of a kind of respect for that innocent child, but those flowers on his desk reminded us of him every day."