Dear Kimberly Tilley,
Thank you for posting your question to History Hub!
The National Archives has Federal case files from the U.S. District Courts. The case file you are asking for was tried in the Missouri Superior Court for the County of Clinton. You will need to contact the Clerk's Office of that Court to determine where their records are, if they are still in existence. You could do the same for Andrew County. In addition, If you haven't done so already, reach out to the Missouri State Archives (https://www.sos.mo.gov/archives ).
The Pinkerton Detective Agency was not a government agency, but is still in operation today. A company with such a storied history would hopefully have some archival materials to aid in your research. You can try to contact them directly (www.pinkerton.com)
We hope this is helpful. Best of luck with your research!
Thank you so much, Eric! This is very helpful!