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.
Who was Dietz impersonating when the FBI investigated him?
had taken on the identity of James One Star, an Oglala man of the Pine Ridge Indian Reservation 12 years his senior who had disappeared in Cuba.
Did Dietz do any jail time for the fraud?
his mother's fallacious testimony (to protect him from prison) resulted in a hung jury, but Dietz was immediately re-indicted.
The second trial resulted in a sentence of 30 days.
Did he have any other Indian brothers and sisters from the mother that testifed for him?
CANNOTANSWER.
Where was the trial for his fraud held?
Dietz was tried in Spokane, Washington in June 1919 for the first offense.
Was Dietz married to anyone else during this time?
Dietz divorced De Cora in November 1918, charging her with abandonment.

Question: What tribe was he claiming to be a member of?
Self-contained rewrite: What tribe was  William Henry Dietz claiming to be a member of?