Learn English – “at the last” referring to in this sentence written by H.P. Lovecraft

This is quoted directly from H.P. Lovecraft's short story, "The Dreams in the Witch-House":

Nor was any spot in that city more steeped in macabre memory than the gable room which harbored him–for it was this house and this room which had likewise harbored old Keziah Mason, whose flight from Salem Gaol at the last no one was ever able to explain.

Can someone explain what "…at the last…" is referring to in this instance?

Here is the full paragraph:

He was in the changeless, legend-haunted city of Arkham, with its clustering gambrel roofs that sway and sag over attics where witches hid from the King’s men in the dark, olden days of the Province. Nor was any spot in that city more steeped in macabre memory than the gable room which harboured him—for it was this house and this room which had likewise harboured old Keziah Mason, whose flight from Salem Gaol at the last no one was ever able to explain. That was in 1692—the gaoler had gone mad and babbled of a small, white-fanged furry thing which scuttled out of Keziah’s cell, and not even Cotton Mather could explain the curves and angles smeared on the grey stone walls with some red, sticky fluid.