The Mishna on today’s daf closes with a statement of Rabbi Meir that whenever there is a tradition that ritual impurity is found in a given place, we must assume that it is there until it is located and removed. The Sages argue that once a thorough search is performed, even if the source of the impurity is not located, we can assume that it is not there.
The Gemara follows this discussion with a number of stories. In each one the source of the impurity is found only after a search had been completed, but the Gemara explains that in each case the search must have been inadequately performed. One of the stories relates to an incident at the close of the First Temple period.
Abba Shaul taught that a large area at Bet Horon was reputed to be unclean, but an elderly man named Rabbi Yehoshua bar Hananiah knew of a way to clarify the matter, and he located a large pit full of bones. The Gemara continues:
That was the pit which Yishma’el the son of Netaniah had filled with slain bodies, as it is written, “Now the pit wherein Yishma’el cast all the dead bodies of the men whom he had slain by the hand of Gedalia.” But was it Gedalia that killed them? Was it not in fact Yishma’el that killed them?— But owing to the fact that Gedalia should have taken note of the advice of Yohanan the son of Kare’ah and did not do so Scripture regards him as though he had killed them.
The story of Gedalia’s murder at the hands of Yishma’el ben Netaniah is described at length in Chapters 40-41 in Sefer Yirmiyahu. After the destruction of the first Temple and the exile of the Judean leadership, the Babylonian king appointed Gedalia as governor of the remnant of Jews who remained in the Land of Israel. It appeared that even with the Temple destroyed, there was still a future for the Jewish people in their homeland. With the encouragement of King Balis of Amon, Yishma’el ben Netaniah assassinated Gedalia, who had been warned by his ally Yohanan ben Kare’ah that he was marked for death by his enemies.
Gedalia’s death was the final blow to the Jewish community, which chose to flee to Egypt, leaving the Land of Israel desolate. In commemoration of this event, the Fast of Gedalia was established on the day after Rosh HaShanah.