Ukrainian prosecutors have launched an investigation into an alleged Russian execution of a Ukrainian soldier found dead with a sword inscribed with “for Kursk” in his body, in an apparent act of revenge for Kyiv’s recent incursion into the Russian border region.

The village is 20 miles west of the main Ukrainian salient in Russia’s Kursk Oblast. Six weeks ago, a powerful Ukrainian force—around a dozen battalions from eight or so experienced brigades—invaded Kursk and quickly gained control of 400 square miles of Russian soil.