This infamous criminal case is one of the most cited examples of "anti-Italianism" in America. In 1927, Nicola Sacco and Bartolomeo Vanzetti were executed for the murders of a paymaster and a security guard during an armed robbery in Braintree, Massachusetts. There's still debate about whether they were guilty or innocent, and it's possible that one was guilty and the other wasn't, but I haven't studied the case closely, so I don't really have an opinion either way. However, few people deny that they received an unfair trial compromised by tainted evidence, and there were worldwide protests over their convictions and executions, including by prominent figures of the day like Albert Einstein, George Bernard Shaw, H. G. Wells, Upton Sinclair, John Dos Passos, Dorothy Parker, and Edna St. Vincent Millay.
While it's possible that their status as immigrants from Southern Europe could have been a factor, since it was a time in American history when foreigners and non-Northern Europeans were looked at suspiciously and unfavorably, I've seen no credible evidence of that, let alone that specifically anti-Italian prejudice played any part. What's certain is that they were anarchists and followers of Luigi Galleani, a notorious revolutionary who advocated violence, assassination and bombing. Militant anarchists were a big problem back then, kind of like militant Islamists today. President William McKinley had been assassinated by Polish-American anarchist Leon Czolgosz in 1901, and the Department of Justice had arrested and deported 500 anarchists and other alien radicals in the 1919 Palmer Raids, including Galleani and eight of his followers. So any prejudice against Sacco and Vanzetti would have been based on their political affiliations and activities more than anything else. Indeed, the judge at the trial, Webster Thayer, was an outspoken opponent of anarchism and bolshevism who made several inappropriate off-the-bench comments to that effect, for which he was criticized by the press and his peers.
But that attitude was common at the time, and the Sacco and Vanzetti case in fact has a historical precedent that's almost identical in every detail, except that everyone involved was of Northern European descent. In 1886, there was a bombing during a demonstration at Haymarket Square in Chicago, and eight anarchists (five German immigrants, a German-American, an Englishman, and a Southerner) were tried and convicted as conspirators in the death of a police officer, despite a lack of evidence against them. Four of the men were hanged, and a fifth committed suicide in prison on the eve of his scheduled execution. Their unfair treatment also generated worldwide protests, and it's thought by most that they were likely innocent. To this day, no one knows for sure who the bomber was, but speculation revolves around a number of other people, including many who weren't involved in the anarchist movement.
When you look at these parallels and the historical context, it's difficult to imagine that there was anything anti-Italian at play with Sacco and Vanzetti, but some people see only what they want to see.