MELISSA BLOCK, Host:
NPR's Serri Graslie reports.
SERRI GRASLIE: Thomas Lowry is the retired psychiatrist who discovered the pardon. He told the story to MORNING EDITION host Bob Edwards in 1998.
THOMAS LOWRY: Unidentified Man: With just hours to live himself.
LOWRY: Unidentified Man: Wow.
GRASLIE: Lowry and his wife discovered the document while researching hundreds of the president's pardons. Ed Steer is the Lincoln biographer who knows Lowry. And he says fellow historians were excited about the find.
ED STEER: We sort of had this morbid interest in every moment of his last moments. And it is so typical of Abraham Lincoln that among the last things that he did, presumably, was to pardon a soldier.
GRASLIE: The National Archives put the pardon on public display in 2000. Archivist Trevor Plant would show the original to visitors and he says he didn't think twice about the date for years.
TREVOR PLANT: And the more I used it, the more it just - it just didn't look right. The five was a little bit darker, which in itself, isn't, you know, necessarily a red flag, but it looked like there was a number underneath it and I kind of got this gut feeling that something wasn't right here.
GRASLIE: Plant did some research which gave him reason to believe that the pardon's issue date was actually 1864, not 1865. He handed his information over to the Archives' inspector general's office, which began an investigation. He says they emailed Lowry asking for help.
PLANT: The more it became apparent what they were working on, then he stopped writing back.
GRASLIE: Serri Graslie, NPR News.