Item (direct transcription):
What did the Boston bombers do that Hitler didn’t?
End the race.
The informant read the joke on 9GAG, an online social media site.
The informant made it very clear that he would only tell the joke to someone he knew very well and was confident wouldn’t be offended.
This joke fits the common pattern of jokes forming in response to tragic events. In this case, the effect is double, because the joke makes fun of tragedy of both the Boston Bombing and the Holocaust.