Jeremiah 41:2-3

41:2 Ishmael son of Nethaniah and the ten men who were with him stood up, pulled out their swords, and killed Gedaliah, the son of Ahikam and grandson of Shaphan. Thus Ishmael killed the man that the king of Babylon had appointed to govern the country. 41:3 Ishmael also killed all the Judeans who were with Gedaliah at Mizpah and the Babylonian soldiers who happened to be there.

Jeremiah 41:7

41:7 But as soon as they were inside the city, Ishmael son of Nethaniah and the men who were with him slaughtered them and threw their bodies in a cistern.