An Army sergeant went on a deadly shooting rampage at a frontline border outpost last month after being ignored and bullied by his superiors and comrades, military police said Tuesday.

 Announcing the results of its one-month probe into the rampage by the 22-year-old sergeant, identified only by his surname Lim, that killed five people and wounded seven, the military police also said Lim is suspected of planning the alleged crime for about four hours and committed it on his own.

On June 21, Lim detonated a grenade and fired on his comrades at the outpost near the tensely guarded inter-Korean border. Two days after fleeing the scene with his rifle and a stash of ammunition, he was captured after a botched suicide attempt.

"Sgt. Lim planned the shooting rampage at around 4 p.m. that day after being enraged by caricatures his fellow soldiers drew of him on the back of the patrol log," said Brig. Seon Jong-chul, chief of the Army police.

"Though Lim was not the only person who was being made fun of with such drawings, they had been a source of agony for Lim," he added.

He was described "as such characters as Sponge Bob, which is being off the wall and naive," according to Seon.

"At the time of the rampage, Lim also aimed at specific colleagues, rather than shooting at random," the chief said. 

The suspect also told investigators that he "cannot live even after being discharged and had originally planned to take his own life after killing his comrades."

Lim had been scheduled to be discharged in September.

Earlier this month, Lim was formally arrested on multiple charges including murder, attempted murder, and stealing military equipment. (Yonhap)