Conversation so far:
What about Dietz's heritage was contested?
Dietz's Indian heritage was first contested in 1916 after former neighbors who settled on the Pacific Coast heard he was posing as an Indian.
Was he living on an Indian reservation or had he before?
In December 1918 the Federal Bureau of Investigation looked into his heritage after he fraudulently registered for the draft as a "Non-Citizen Indian" with an allotment.

Question: Who was Dietz impersonating when the FBI investigated him?
Self-contained rewrite: Who was William Henry Dietz impersonating when the FBI investigated William Henry Dietz?