The South Korean military says a young marine who shot and killed four of his colleagues Monday had shown signs of mental illness before the incident.

The 19-year-old corporal, identified only as Kim, also wounded one other marine when he opened fire at a barracks on Gangwha island, west of Seoul. He has been hospitalized with wounds from a grenade blast in an apparent suicide attempt.

Captain Kwon Young-jae, the lead investigator in the incident, told reporters Tuesday Kim left behind a note lamenting his life, describing himself as a “troublemaker” and complaining that people have tried to change him.

Kwon also said the young corporal reeked of alcohol before the shooting, suggesting he may have been intoxicated.

Monday's shooting rampage is the worst in the South Korean military since 2005, when eight soldiers were killed by a soldier who threw a grenade and opened fire at a frontline army outpost.

The incidents have raised questions about the level of discipline in the South's 650,000-strong military force, which is largely made up of young men who are required to serve two years. Critics have said the men are abused and harassed.