How would you go about tracking records for inmates listed as being in a detention center/prison in 1940, in Snohomish, WA?
If no responders who have specific knowledge of your "quest" on this community board,
... maybe try ....
reaching out to "corrections" main office staff, if haven't already, with same question. https://snohomishcountywa.gov/178/Corrections
to explore Family Search collections, maybe try "Search" "Research Wiki" for the county. https://www.familysearch.org/en/wiki/Snohomish_County,_Washington_Genealogy
Cyndi's List used to be pretty useful for optional on line databases. https://www.cyndislist.com/categories/
good luck in your search.