An Army reservist behind a deadly shooting at a Seoul training camp had long suffered from mental disorders and it was a premeditated attack, military officials said Thursday.

On Wednesday, the 23-year-old man, surnamed Choi, opened fire on fellow reservists during a shooting exercise at the compound in southern Seoul, killing two and wounding another two. He then shot himself to death.

Citing the results of a probe so far, the Army said the shooter had planned to use the training session as the chance to hurt others and take his own life.

"He had sent some 10 text messages to his friend since March which indicated that he would commit suicide during the reservists' exercise," said Col. Lee Tae-myung, the chief of the Army's central investigation team.

"For one, he told the buddy on April 22, 'I will die on May 12.

Bye,'" he said. The shooting session was part of a three-day exercise that began on Tuesday.

The investigator also pointed to his suicide note as grounds for its interim call that the killings were premeditated.

"The shooting drill is due tomorrow. I want to kill them all and commit suicide," the shooter said in a two-page suicide note found later in his trouser pocket. "I don't know what the purpose of living this life is ... I hate everything about myself, my pride, and my external and internal things. I want to die."

Prior to the beginning of the shooting drill, he also asked his instructor to take the leftmost shooting lane out of the total 20, which would allow him a clearer line of fire for the shooting rampage, according to investigators.

His medical records also showed that Choi received treatment for mental disorders three times in 2010 before joining the Army, and he again went to the doctor three times after being discharged from the military in 2013.

"We've learned that he had been under stress in recent months after failing to obtain a certificate for welding vessels earlier this year," the colonel said.

As an active-duty soldier, he had difficulties adjusting to military life, and was put on the military's "list of soldiers requiring special attention."

As part of efforts to better manage the troops, the military classifies at-risk soldiers into three categories of A to C, factoring in the results of enlistees' psychological consultations and their backgrounds, with A deemed the most vulnerable.

"As a Class B soldier, he was subject to extra care by his senior colleagues. Due to his maladjustment, however, he had been moved from one unit to another four times during active-duty service," another Army officer said.

In the note, the shooter said, "I should have killed everyone (while serving the duty) at the general outpost and committed suicide. It is really regretful that I missed the chance." He was last affiliated with the Army division in the inter-Korean border town of Yeoncheon. (Yonhap)