Gesshin-In temple
The exact details of the betrayal and the executions are unknown, however it is speculated that Saito Hajime, a captain of the Shinsengumi charged with routing out spies, was sent to infiltrate The Guardians of the Imperial Tomb. This however may be a conflation of his duties to ferret out spies within the Shinsengumi. Whether or not he was charged with spying on traitors is not within credible historical records.
Make a reservation in Kyoto Samurai and Ninja Museum