The local electron transport properties of thin aluminum oxide layers used for magnetic tunnel junctions were studied in situ by scanning tunneling microscopy (STM) and spectroscopy under ultrahigh-vacuum conditions. The STM images of the oxide films reveal a granular structure, down to atomic resolution. A reversible switching of the conductive properties of grains, attributed to a charge redistribution, is observed during scanning. We demonstrate the possibility of intentionally switching a grain to the low-resistance state by exposing it to a high current density. We conjecture that the observed switching behavior may be considered as the precursor of an electric breakdown in tunnel junctions.