He was at the United States Federal Penitentiary at Leavenworth Kansas established 1903. The Military Prison was on the Fort Leavenworth grounds known as the United States Disciplinary Barracks (or USDB) Ancestry,com has it mistakenly cited as the township of "Fort Leavenworth" in index and it is just the city of Leavenworth. No where on the the census page has the word "Fort" on it, just Leavenworth County, Kansas and the name "United States Penitentiary" written on "Institution" line.
All federal records, either court, military or prison would be kept at the NARA. www.archives.gov
Found this page with a search of site ....federal prisoner records...
If Sarah was from Romanian too, she became a citizen when John did. (unless her father became one while she still a minor).John became a citizen August 26, 1919. They married 2 days after he was discharged form Military service May 29, 1919. She may have not ever really divorced him as I think it would affect her citizenship. He must have committed a federal crime of some sort. But I would think if he did he would have lost the citizenship and deported after his release.