Jefferson is a member of Vigilance who has been working with Peter Collier since 2010. She was there when Collier made a statement by killing one of of the group's members, who turned out to be working for the FBI. (“Deus Ex Machina”)
At Vigilance's kangaroo court Jefferson acted as the judge, and witnessed Rivera get killed after he failed to answer Collier's questions about the Machine. She also heard the others' testimonies, including Finch's statement that he was the creator of the Machine.