After a time, Himmler found that the killing methods used by the "Einsatzgruppen" were inefficient: they were costly, demoralising for the troops, and sometimes did not kill the victims quickly enough. Many of the troops found the massacres to be difficult if not impossible to perform. Some of the perpetrators suffered physical and mental health problems, and many turned to drink. As much as possible, the "Einsatzgruppen" leaders militarized the genocide. The historian Christian Ingrao notes an attempt was made to make the shootings a collective act without individual responsibility. Framing the shootings in this way was not psychologically sufficient for every perpetrator to feel absolved of guilt. Browning notes three categories of potential perpetrators: those who were eager to participate right from the start, those who participated in spite of moral qualms because they were ordered to do so, and a significant minority who refused to take part. A few men spontaneously became excessively brutal in their killing methods and their zeal for the task. Commander of "Einsatzgruppe" D, SS-"Gruppenführer" Otto Ohlendorf, particularly noted this propensity towards excess, and ordered that any man who was too eager to participate or too brutal should not perform any further executions.