There is no way to know who really threw it. Propaganda of the deed was a very popular tactic at the time, it could have been another anarchist, the police, someone paid by factory owners, who knows. The 8 people above were selected because they were influential people in the anarchist movement not because of any evidence so it's safer to say who it wasn't - the 8 who were found guilty. I'm sure in the act of framing them, any evidence available was destroyed.