Shortly thereafter, on 10 June, he headed the group (Albino Volpi, Giuseppe Viola, Augusto Malacria and Amleto Poveromo) that carried out the killing of Giacomo Matteotti, possibly on the orders of Cesare Rossi, to whom he was assistant at the time.

While Farinacci declared himself "honored" by the task, he could not prevent Dumini from receiving a five-year prison sentence, of which he only served eleven months, benefiting from an amnesty ordered by Mussolini.

Freed in 1927, Amerigo Dumini left for Italian Somaliland, having been awarded a large state pension (5,000 lire).

Meanwhile, he warned General Emilio De Bono that he had filed a manuscript detailing Matteotti's murder with notaries in Texas.

Dumini remained in the region for more than a decade and was captured by the British Army at Derna during the North African campaign of World War II.

Sentenced to death as a spy, he was hit by 17 bullets from a firing squad and still managed to remain alive, escaping to safety in Tunisia during the night.