Answer:
"One Galleanist in particular, Italian anarchist Mario Buda (1884–1963), an associate of Sacco and Vanzetti and the owner of a car which led to the arrest of the latter for a separate robbery and murder, is alleged by some historians, including Paul Avrich, to be the man most likely to have planted the bomb. Avrich and other historians theorize that Buda acted in revenge for the arrest and indictment of Sacco and Vanzetti."
I looked it up, there is an article about it.