The ion-tearing mode instability is studied in a magnetotail configuration, with an embedded thin current sheet at the center of the plasma sheet, taking into account the effects due to electron dynamics and perturbed electrostatic potential. It is found that the presence of trapped electrons in the forced current sheet has a strong stabilizing effect on thinner current sheets. The ion-tearing instability can exist provided the trapped electron population density is less than about 10--20% of the total electron number density. This can be achieved either by the background turbulence present in the plasma sheet or by the high-frequency plasma instabilities excited by the strong current of the inner current sheet itself. Under such a situation, the configuration becomes progressively more ion-tearing unstable as the forced current sheet becomes thinner. This eventually could led to the disruption of the current sheet and to the onset of substorm expansion phase. ¿ American Geophysical Union 1993