Defense Minister Kim Kwan-jin has ordered military commanders to inspect the standards of discipline and irregularities, following a shooting rampage earlier this week by a marine that killed four colleagues on a front-line unit, a source said Saturday. Questions over discipline have been raised since the 19-year-old marine corporal, only identified by his surname Kim, opened fire on Monday at his base on Gwanghwa Island near the tense maritime border with North Korea. Military authorities found that Kim, who was found to be mentally unstable at the time of the shooting, was angered by alleged bullying and beatings by his colleagues. "Defense Minister Kim issued an emergency directive yesterday, instructing all military commanders to inspect possible irregularities at their bases," the source said on the condition of anonymity. The commanders were ordered to report their findings by the end of August, the source said. The shooter was hospitalized after unsuccessfully trying to kill himself with a grenade. Military investigators found some notes inside Kim's lockers. The notes read, "Many people have tried to change me. That's how much of a troublemaker I've been. ... To tell you the truth, I want to give up on everything and put an end to this all." Two years of military service is compulsory for all able-bodied South Korean men.