A rare 1796 copy of Immanuel Kant’s Perpetual Peace: A Philosophical Sketch was gifted by French President Emmanuel Macron to Pope Francis in front of a private audience on Monday. Then, amid the fanfare, Poland’s foreign ministry said it was investigating whether the volume was looted while the country was occupied by the Nazis during World War II.

After photographs of the book were posted online, viewers pointed out a stamp from a Polish reading society founded by university students in Lviv (Lwow in Polish), now part of present-day Ukraine. The stamp’s presence prompted further investigations by the Polish foreign ministry.

“The Foreign Ministry is investigating the circumstance of French President Emmanuel Macron’s gift to Pope Francis,” foreign ministry spokesman Lukasz Jasina told the Polish Press Agency on Tuesday.

French journalist Arnaud Bedat claims that the volume was purchased from a rare books specialist in Paris. The contested work has been in France since around 1900, according to the store’s website.

Additionally, there was evidence of a stamp from a Parisian bookseller, who was in operation at the end of the 19th century.

Claims like these are not uncommon, as German forces in Poland looted thousands of cultural artifacts during the war. Earlier this year, for example, France returned 15 Nazi-looted artworks including paintings by Gustav Klimt and MarcChagall.