CAIRO — Egypt launched an investigation Monday of the country’s former military rulers and their alleged role in the killing of protesters during their 18 months in power, an unprecedented civilian probe into the affairs of an army that has traditionally shielded itself from outside scrutiny.

International and local rights groups have pressed Egypt’s newly elected president to hold to account the council of military officers who ruled the country from the February 2011 overthrow of Hosni Mubarak to this summer. At least 120 protesters died in clashes with security forces and soldiers during this time.