Young Irish immigrant Patrick Prendergast is described by those who know him as a "hapless and sorry" human being (1.4.41). Oof. That's not how anyone wants to be described.
He works at the Chicago Inter Ocean newspaper with the mail boys, and when he's not working he writes scores of postcards, perhaps hundreds at a time, to powerful men in the city of Chicago. The guy is unstable, to say the very least.
Though he's clearly a "troubled young man" (1.4.43), he's deemed harmless. He targets one victim, Carter Harrison, and operates on the delusion that he'll help the mayor secure his fifth term and be rewarded with a city government position.
Of course, Harrison has no knowledge of Prendergast's efforts, so when he's elected, he gives no thought to Prendergast. Not very happy about this, Prendergast arrives at Harrison's doorstep, shoots him, and then turns himself in to the police.
He tells the cops that Harrison betrayed his confidence and didn't live up to his word. He also tells them:
"Lock me up; I am the man who shot the mayor." (3.21.28)
Guess which of Prendergast's statements the police take at face value?