Hi Kathryn -- thank you for posting to History Hub!
Citation #206 in the article you linked to points to report #23 from the U.S. House of Representatives in the 2nd Session of the 39th Congress. The report is titled Murder of Union Soldiers and it was ordered published on Feb. 22, 1867. The report can be downloadable via ProQuest Congressional, a subscription database with the full text of the Serial Set, or it can be read online via Google Books.
The report was issued by the Select Committee to Investigate the Charges for Murder of Union Soldiers in South Carolina. I checked our finding aid for records of the House of Representatives in the 39th Congress and it looks like we do hold some committee papers from this select committee at file code HR39A-F28.2 -- you can view the corresponding entry in the National Archives Catalog.
If you'd like to pursue accessing these records please email us at firstname.lastname@example.org. Please note that National Archives research rooms are currently closed due to the pandemic. More information about NARA's operating status is available on our website.