With its stacks of yellowing historical documents and staff of earnest archivists and librarians, the National Archives doesn’t seem like a typical setting for intrigue. So workers at the Philadelphia branch have understandably been shaken by a whodunit that has unfolded in their normally placid corridors during the last few months.

The unusual crime began to unravel last September, when Dean Thomas of Gettysburg, Pa., had the sensation of déjà vu while reading an eBay offer for three Civil War documents from 1861 and 1862 that his brother was bidding on for him. Thomas, who publishes Civil War and American Revolution history books, got up from his desk and looked into one of his many black binders, the one that holds the letters he photocopied some 20 years earlier at the Philly National Archives. There, he found the same ones he was seeing on eBay, being sold by a private seller, “hchapel.” His brother won the bid on a Sunday night and the purchase went through for $298.88. Payment was to be delivered to Denning McTague of Philadelphia; his full address and contact information clearly printed. “It wasn’t very secretive,” says Thomas.

The next morning he called up an acquaintance at the archives and asked if there was a sale of original documents under way. There wasn’t, so Thomas called up the Archives’ Inspector General’s office. A quick search revealed that 164 documents were missing, and soon the National Archives Archival Recovery Team (ART) got to work. Like most of McTague’s other eBay clients, Thomas quickly returned his letters. Only three documents are still missing.