Begin your search using the Discovery box. This time select two of the keywords you found in your background reading. For example, you could begin your search using the keywords gangs and firearm or guns. When you click enter or search you will see a list of sources.
You can use the left sidebar to limit your search by:
Also take note of the "subjects" limiter. Each article has a list of words that have been tagged to the article. These are "official" terms that work something like hashtags in Twitter. These subject words are a great source of additional keywords to test in your searches. Since the official subject term used for firearms sales is firearms trafficking, the search term could be changed to firearms trafficking.