[1] After failing to prevent the assassination attempt on Mussolini by Gino Lucetti, Crispo Moncada was replaced as chief of the police by the prefect of Genoa, Arturo Bocchini, with whom Leto established a strong professional bond.

He took part, together with Michelangelo Di Stefano, in the investigation on the attempted assassination of King Victor Emmanuel III in 1928, a bomb attack at the Milan Fair that had resulted in twenty deaths and forty wounded.

The investigation led to the arrest of several members of the Justice and Freedom movement, including Umberto Ceva, who was accused to have prepared the bomb and who committed suicide in prison, Ernesto Rossi and Riccardo Bauer, who in 1931 were sentenced to twenty years' imprisonment by the Special Tribunal for the Defense of the State, although their responsibility in the attack was never proven.

In December 1943 he directed an operation in Rome, carried out by Erich Priebke and Pietro Koch, that resulted in the arrest of eighteen anti-Fascists, including Communist Giovanni Roveda.

In March 1944 he moved to Valdagno, bringing with him a substantial part of the OVRA archive (6,000 boxes of documents) which was housed in the premises of the wool mill owned by industrialist Gaetano Marzotto.

On 12 April 1946 he was acquitted in the Court of Appeal from the charges brought against him; according to historian Mimmo Franzinelli, he exchanged his immunity for the destruction of documents that implicated members of anti-Fascist parties in collaboration with the regime.