I will post the picture of the Obit below, it says my 3rd Great Grand father Frank Ziegler was a U.S. Marshall in Indian Territory. He was in the U.S. Cavalry during the Civil War between 1861 to 1862. He then moved west to Kansas and married Eleanor Bourassa in 1871. His wife was a Potawatomi Indian, and at some point they moved to Indian Territory in OKLA.

How do I find out if he was a U.S. Marshall around the dates of 1870's &1880's in OKLA?

Thanks for any help!