The sergeant who killed five soldiers in a shooting rampage last month said he was treated as an “invisible man” in his frontline unit of the Army’s 22nd Division, military investigators said Monday.

The claim by the 22-year-old sergeant, surnamed Lim, reinforced the speculation that bullying was a cause of the incident. The bereaved families have responded angrily to the speculation, saying the victims are being treated as if they were to blame.

“Sgt. Lim said during our investigation that his colleagues treated him as if he did not exist,” said a senior military investigator during a meeting with reporters.

The investigator also said that Lim’s senior colleague smacked the back of his head for an unspecified reason on the day of the shooting, and that Lim saw a picture his fellow soldiers drew of him, depicted as an ugly skeleton.

Weak, short and slightly bald, Lim was mocked at by his colleagues, who called him “slime” or “old man.”

Lim is currently recovering from a self-inflicted gunshot wound in a military hospital in Gangneung, Gangwon Province. The investigative authorities plan to seek an arrest warrant as early as this week as they monitor his heath conditions.

The authorities said that the investigation could not proceed efficiently as Lim continually complained of fatigue, pain and other health problems.

President Park Geun-hye on Monday expressed her apology for the shooting spree.

“I feel deeply sorry at the thought of the parents who may feel anxious about their sons serving in the military, due to such incidents that have happened continuously in the military,” she said during a meeting with her senior secretaries.

“The military should not forget its duty to send the enlisted soldiers back to their families safely after their service.”

Park also directed the military to thoroughly verify the cause of the shooting incident and craft measures to prevent a repeat of the deadly incident.

Meanwhile, the 119 state emergency agency and military came under fire for flawed communication, which caused a delay in dispatching an emergency helicopter to the frontline unit where the shooting took place on June 21.

The 119 agency sent a helicopter to the site upon request from the military, but it could not land on the military helipad as it was not authorized to enter the site. The agency sought to gain access approval, but due to flawed communications, a delay of some 50 minutes occurred.

By Song Sang-ho (sshluck@heraldcorp.com)