Who writes the user stories?

In agile design/development, who writes the user stories and what is the process? Should UX researchers do their research first in order to help define better user stories?

In my current project, we have user stories, but UX has not been involved in user research to help define those stories. This has taken us down some inefficient paths with the project. I'm not experienced with UX strategy, but it seems a wise approach would be to base user stories on upfront UX research. Yes? No?