1. Lack of Concrete Evidence: Although the prosecution presented physical evidence, such as the pistol found near the crime scene, these pieces of evidence were circumstantial at best. The defense argued that the evidence gathered was weak, inconclusive, and biased, especially in light of expert testimonies that challenged the prosecution's interpretation of the physical evidence.
2. Anti-Immigrant and Anti-Anarchist Sentiments: During the trial, public sentiment was heavily influenced by xenophobic attitudes towards immigrants and anarchists. The defendants, Sacco and Vanzetti, were both Italian immigrants, and their convictions seemed to be rooted in prejudices and fears of political activism and radicalism rather than compelling factual evidence.
3. Allegations of Improper Conduct: The trial raised concerns about the impartiality and potentially manipulative tactics of Judge Webster Thayer. Allegations surfaced that Thayer harbored strong biases against immigrants and anarchists even before the trial began. His comments during the trial and seemingly disproportionate treatment of the defense's arguments and witnesses caused some to believe that he was not a neutral arbiter but rather favoring the prosecution.
4. Jury Irregularities: The jury selection process became questionable, with the defense discovering that some jurors held preconceived notions of guilt or were hostile towards the defendants due to their immigrant backgrounds. Additionally, the jury's deliberations were highly unusual. Instead of following standard practices and considering the case rationally, the jury members reportedly engaged in emotional and personal discussions, further fueling doubts about the impartiality of the verdict.
5. Extensive Public Protest and Advocacy: The case attracted significant public attention and sparked an outcry among labor unions, intellectuals, and activists who believed that Sacco and Vanzetti were targeted for their political beliefs and not for concrete, verifiable evidence. As the case developed, international protests and campaigns emerged, calling for a reevaluation of the trial and highlighting its problematic aspects.
6. Lack of Clemency: The verdict found the defendants guilty of murder, and despite numerous calls for clemency and the doubts raised about the trial's fairness, both the trial judge and the higher court upheld the conviction and death sentences. This lack of responsiveness to the concerns and evidence presented led to further outrage and controversies surrounding the case.
In essence, the case remains controversial because of the combination of circumstantial evidence, biases against the defendants' origins and beliefs, questionable judicial conduct, irregularities in the jury process, and the absence of clemency based on the doubts surrounding the verdict's fairness.