Conversation so far:
When did Robert Mueller start a law career?
1973
What did Robert Mueller do for the firm Pillsbury, Madison and Sutro?
Robert Mueller worked as a litigator at the firm Pillsbury, Madison and Sutro in San Francisco.
What did Robert Mueller do after working as a litigator for Pillsbury, Madison and Sutro?
Robert Mueller then served for 12 years in United States Attorney offices.

Question: What did he for the US Attorney?
Self-contained rewrite: What did Rober Mueller do for the US Attorney?