For any investigator, finding the common denominators or "links" between gangs or singular subjects is an intricate part of any investigation. While to the average person, some of these may seem nebulous, the fact is that these common "links" can provide vast amounts of needed investigation to establish what significance the "link" represents.
In this 1935 memorandum, some of these links between the Barker/Karpis Gang and the Dillinger Gang are described. You can see those associations here.