Are you talking about the mission with the murderer?
Nah, the murder mission is Windhelm I think. The mission I'm talking about in Riften is the Scooma mission... There's this woman who is hooked on Scooma and you help her and then the Jarl asks you to take out whomever it is that's smuggling the Scooma in.
For whatever reason I can't complete part of the mission that requires me to read a letter that I supposedly found. I somehow don't have that letter in my inventory though so the mission can't be completed. When I put on the mission indicator it sends me back to the place I'm supposed to get the letter but that satchel is empty when I go to it.