On September 4, 1918, someone tossed a bomb into the Adams Street entrance of the Chicago federal building and post office, killing four, including postal workers Edwin Kolkow and William Wheeler, and injuring dozens of others. The police focused their attention on members of the labor organization known as the Industrial Workers of the World (IWW). Officers rounded up and arrested almost 100 members of that group in the early days after the attack; all but a handful were released over the next few days. Despite intense investigations, no convictions ever came from the attack, nor a final reason for the bombing. On a side note, one of the many postal workers in the building at the time was a substitute letter carrier named Walt Disney.