Very good job so far guys, but you're missing some important things. Firstly, they do know the drug lord's assistant, and it's not Mason, but I left that out on purpose, because the assistant is irrelevant.
Secondly, to answer Smiley, the 2nd death is also irrelevant, HOWEVER, there is a big clue that can find you the killer.
Thirdly, we need proof and connections. Smiley figured out a part of clues, but not entirely. Let me help: it is connected to the first crime scene. There is not a single indication about anyone "forgetting" anything in the crime scene. You just need to pay more attention to the details, than look for more things that are not there. Everything you need is in the case, if something is not mentioned, it simply means it's not necessary.
And let me give you a small, very important hint: someone helped the killer a little, so there are 2 suspects, BUT, 1 killer.
Good luck.