SEOUL >> A South Korean marine expressed anger about being shunned and slighted and showed signs of mental illness before he killed four colleagues at a front-line base, military officials said Tuesday.

The 19-year-old corporal, surnamed Kim, also wounded another marine Monday when he opened fire at a Ganghwa Island base just south of the tense maritime border with North Korea, officials said.

Chief investigator Kwon Yeong-jae told reporters Kim had a “psychological problem” and referred to a note in which Kim was said to have lamented the state of his life. Kim was also monitored by superiors due to worries that he might cause trouble, Kwon said.

Kim tried unsuccessfully to kill himself by detonating a grenade after the shooting, Kwon said.

Another military official, who declined to be named because Kim was still being questioned, later said Kim wrote in a memo to an investigator that his fellow marines had treated him with little respect.

The official said the situation lasted for a while and fellow marines found Kim’s behavior to be a problem.

According to a defense ministry report obtained by the office of opposition lawmaker Shin Hak-yong, Kim showed “anxiety, personality disorder and schizophrenia” in psychology tests during his training as a recruit. Kim’s colleagues said the corporal was “hot-tempered and unstable,” the report added.

A defense official, who also would not be named because of office policy, said one of Kim’s memos to investigators read: “Ostracizing must disappear.” The official would not give further details.

All able-bodied South Korean men must serve about two years in the military under a conscription system aimed at countering aggression from North Korea. Marines, however, volunteer for service and are considered the toughest branch of South Korea’s 650,000-member military.

Kim’s base is one of a cluster that sit on islands on the inter-Korean front line, and tensions there have taken a toll.

The waters are claimed by both countries. Boats routinely jostle for position during crab catching season, and three deadly naval clashes since 1999 have taken a few dozen lives.

Since the North bombarded a South Korean island near Ganghwa last year, the South has beefed up its high-tech weapons systems and the marines’ combat readiness.

The shooting is the worst in South Korea’s military since 2005, when a soldier tossed a hand grenade and opened fire at a front-line army unit in a rampage that killed eight colleagues and injured several others. Pfc. Kim Dong-min told investigators he was enraged at superiors who verbally abused him.

The shootings have raised questions about the level of discipline in South Korea’s military. Kwon said Kim’s base failed to maintain proper security before the attack. Kwon said there was only one person with access to the base armory, instead of the required two.