|The following is based on Van Buren and is not canon.|
Find the poisoner was going to be a quest in Van Buren, the canceled Fallout 3 by Black Isle Studios.
Once the commander is cured she will ask to see the Prisoner. She wants to know who poisoned her and why. This quest is assigned and can be completed through dialogue with various people.
The poisoner will be revealed to be Devon Hill. In dialogue with the quartermaster, the Prisoner will learn of the Circle of Steel.