From the start, there were several investigation hypotheses: from red terrorism (there were some claims done by extra-parliamentary groups, but the Red brigades denied responsibility), to the Mafia. Suspicion on the Mafia was high because the year before in Alcamo Marina there were two murders which occurred within one month of each other: that of Francesco Paolo Guarrasi, assessor of public works of Alcamo and former
DC-affiliated mayor, and that of Antonio Piscitello, the municipal councilor of the town. The investigation was led by the then Carabinieri captain
Giuseppe Russo, who would later be murdered by the
Corleonesi. In the end, the investigation shifted to a group of young people from the area, and resulted in the arrest and conviction of four people: Giuseppe Gulotta, Giovanni Mandalà, Gaetano Santangelo and Vincenzo Ferrantelli. The first two were sentenced to life in prison, whereas Gaetano Santangelo (arrested only in 1995) and Vincenzo Ferrantelli to 20 years. A mechanic from
Partinico, considered close to anarchist circles, Giuseppe Vesco, confessed to the massacre and accused the four youngsters, only to retract his testimony immediately afterwards: he was found mysteriously hanged in his jail cell some months afterwards, despite having only one hand. Mandalà died of natural causes after years of jail in 1998, whereas Santangelo (until 1995, when he was arrested) and Ferrantelli, from an appeal to the next, fled Italy to Brazil and obtained the status of refugees there. The folder containing the documents on Alcamo Marina done by Impastato was taken by the Carabinieri (who at the time accused Impastato of dying while trying to set a bomb on train tracks, whereas in reality, he was killed by mafiosi on the orders of
Gaetano Badalamenti) in the house of his mother Felicia Impastato shortly after Peppino's death, and it was never given back to the family unlike the other documents (as explained by his brother Giovanni).
Walter Veltroni, member of the Commissione Parliamentare Antimafia, stated that the perpetrators of the Alcamo Marina massacre were agents of the
Gladio organization. According to him, the day before the massacre Falcetta and Apuzzo had stopped a van carrying weapons driven by men of the organization, who then assaulted the barracks in order to get them back. Fifteen years after the massacre the police discovered an arsenal belonging to two Carabinieri officers: the corporal Vincenzo La Colla, lead of the escort for the former Culture minister
Vincenza Bono Parrino, at the time president of the Defense Commission of the Senate, and the brigadier Fabio Bertotto (several times involved in missions in
Somalia). They were accused of supplying weapons to the Mafia clan of Alcamo and were revealed to be members of the secret services, and were eventually acquitted. La Colla pleaded guilty to illegal detention of firearms. == New revelations by brigadier Olino ==