There are many sources discussing the idea that the bodies of tzadikim do not impart ritual impurity (tum'ah), allowing Kohanim to be present at their graves. Ultimately, the ruling follows the Maharil's opinion, as evidenced by the practice at the Alter Rebbe’s gravesite in Haditch, where a separating wall was constructed so that Kohanim could visit the gravesite without encountering impurity.