A South Korean Marine went on a shooting spree near the tense North Korean border Monday, killing four colleagues and wounding another before himself being injured in a grenade blast, the military said. Investigations are under way into whether the 19-year-old corporal, identified only as Kim, tried to commit suicide with the hand grenade after his rampage, the military said. Kim opened fire at a Marine Corps barracks on Ganghwa island 37 miles (60 km) west of Seoul. A staff sergeant and three other Marines were killed, while a private was injured and taken to hospital, said Corps spokesman Kim Tae-Un. \"After opening fire with a K-2 rifle, the corporal was found injured in a separate place where a hand grenade was detonated,\" the spokesman said. The shooter\'s injuries were preventing him from answering questions about his motives, he added. It was unclear how many shots were fired, but the spokesman said that Marines in that unit normally load 15 bullets in the magazine of a firearm. The injuries to the private and the shooter were not life-threatening. \"We have to wait until the investigation makes some progress to find out more details,\" the spokesman said. \"Once the shooter is treated, our investigation will get under way in earnest.\" Local resident Lee Yong-Soo told Yonhap news agency there was a sound of firing and then \"four soldiers in their underwear rushed out of the barracks and ran away with all their might\". The agency quoted another resident as saying he heard an exchange of insults among the Marines before the shooting. The elite Marine Corps is charged with guarding \"frontline\" islands in the Yellow Sea near the disputed border with the North. Monday\'s incident will likely raise questions about standards of discipline in the South\'s largely conscripted 650,000-strong military. Eight soldiers were killed and two seriously injured in 2005 when a soldier threw a grenade and sprayed bullets over sleeping colleagues at a frontline guard post north of Seoul. The soldier in the 2005 incident alleged senior colleagues had bullied him. In 2008 an army private struggling to adapt to military life threw a grenade at sleeping colleagues, injuring five. And last month two Marines on another frontline island opened fire at a civilian passenger plane after mistaking the Airbus for a North Korean intruder. The aircraft was not hit. Able-bodied South Korean men must undergo at least two years\' military service, and some complain of abuse and harassment of junior soldiers by their seniors. Marines and other military units have been on high alert along the disputed Yellow Sea border for months. Tensions rose after the South accused the North of torpedoing a warship in March 2010, killing 46 sailors. The North denied the warship attack but shelled a South Korean border island last November, killing two civilians and two Marines. The North\'s military has recently warned of \"merciless\" retaliation for anti-Pyongyang slogans displayed by South Korean troops near the land border. The two countries have remained technically at war since their 1950-1953 conflict ended only with an armistice instead of a full peace treaty.