The only sources if you can find them. Everything else is an opinion.
One of my favorite American “Histories” is “The Gangs of New York” (Aside: Note I have seen similar books for New Orleans, Philadelphia & Chicago. Haven’t read them yet and I am sure its different authors!). It’s an extremely colorful & fascinating read. The DiCapro movie only covers a very small part of the book (and not well!) Now my point the author’s source is all newspaper accounts. Which are problematic since we know their exaggerated & fictionalized to make a good story.
So what do you do when that’s all you have? The best you can & filter with common sense.