Conversation so far:
Where did Robert Mueller go to school?
Robert Mueller studied at Princeton University, New York University, and University of Virginia
Where did Robert Mueller work at?
After receiving his Juris Doctor in 1973 from the University of Virginia School of Law, Robert Mueller worked as a litigator at the firm Pillsbury, Madison and Sutro
What did Robert Mueller do next for work?
Robert Mueller then served for 12 years in United States Attorney offices.

Question: What office did he work at?
Self-contained rewrite: What office did Robert Mueller work at?