There was some dispute at the Haymarket trial whether or not August Spies had been invited to the Lumber Shovers' Union rally or had gone there on his own to stir up trouble. The evidence strongly indicates that the former was the case. Spies himself testified at the trial that he was invited by the union members the evening before, at a meeting of the Central Labor Union at Greif's Hall on West Lake Street. Spies claimed that he attended this meeting in order to cover it for the Arbeiter-Zeitung. He did not want to go to the Lumber Shovers' meeting, he explained, since "having spoken two or three times almost every day for the preceding two weeks, I was almost prostrated." He agreed, nonetheless, to be the German-language speaker at the rally.

Contrary to the testimony of a prosecution witness who claimed that Spies provoked the crowd, as well as to Cyrus McCormick Jr.'s charge that the anarchist leader had been the cause of the riot, Spies maintained that he spoke calmly, advising the workers to stick together. In a remark that the state would use against him and his codefendants to show the anarchists' contempt for the workers they supposedly served, Spies described his listeners as "according to my judgment not of a very high intellectual grade." Once the fighting at McCormick's broke out, the Lumber Shovers' rally fell apart. Spies was on the spot elected to a committee delegated to talk with the bosses, and he made his way to the factory.

In his trial testimony (see the Haymarket Affair Digital Collection for the complete version), Spies explained that what he saw appalled him:

"They were shooting all the while. I thought it must be quite a battle. I don't know whether the gentlemen [of the jury] are acquainted with the locality or not, but in front of McCormick's factory there are railroad tracks and upon these railroad tracks were standing a number of freight cars. The people were running away and hiding behind these freight cars as much as they could to keep out of the range of the pistol firing, of the fusillade; and when I came up there on this prairie right in front of McCormick's, the police—the fight was going on behind the cars, and I was on this side. I saw a policemen run after the people who were flying, who were fleeing, who were running away, and firing at them. Well, as a matter of course my blood was boiling, and I think in that moment I could have done almost anything, seeing men, women and children fired upon, people who were not armed fired upon by policemen."

Spies hastily returned to the Arbeiter-Zeitung offices in downtown Chicago. He then composed what became known at the trial as the Revenge Circular, expressing his anger at what he had witnessed and exhorting workers to rise up against their oppressors. In equally expressive language, he also wrote up an account of the event for the next day's paper, for which the lead headline was "Blut" (blood). The article and an accompanying editorial written by Michael Schwab are in the "From the Archive" section of this Act.