SEOUL (CNN) -

Like most South Korean mothers, Ahn Mi-ja was worried about her son's compulsory military service.

She has a point -- Yoon Seung-joo's medical unit is stationed close to the border with North Korea, which remains a very real threat.

Yet the real enemy was far closer than she could have ever imagined.

For more than a month, Private Yoon was beaten. Denied food, the 20-year old was forced to lick phlegm from the floor or eat his own vomit. He was hooked up to an intravenous drip to revive him when he faltered.

Then on April 6 this year, his body could take no more. Force-fed frozen food as he was being beaten, he simply stopped breathing and died. The official autopsy report stated the cause of death as asphyxiation.

His alleged abusers were half a dozen of his fellow comrades. The oldest was just 25.

Extensive bruising

It was a Sunday afternoon when Yoon's parents got the call to come to the hospital. When Yoon's two sisters arrived they noticed extensive bruising on his body and questioned the claim that he choked.

"The military came and started taking photos," Yoon's mother recalled. "They didn't answer our questions, just took photos and left."

It would take months before the full scale of Yoon's abuse became clear. And it did not come directly from the military, rather from a leaked internal report obtained by a military human rights group.

"The handling of this case is simply not transparent," said Lim Tae-hoon of The Center for Military Human Rights. "We would like the families of the bereaved or the victims to have unlimited access to military information."

Reforms

South Korea's defense ministry has made some structural changes: Parents can now call and visit more often, leave is more flexible and barracks are being updated.

"The Korean military has tried very hard to obliterate violence," said spokesman Kim Min-seok. "But recently the violence has resurfaced. Violence is like a weed, you have to constantly pull it out and some officers probably misunderstood, thinking violence had disappeared."

An army survey conducted in April in tandem with the investigation seems to back up Kim's suggestion the military became complacent about violence. Almost 4,000 previously unreported cases of abuse emerged.

A military dictatorship until the late 1980s, South Korea has struggled with decades of bullying within the military. Now, there are calls for more transparency. The defense ministry rejects all accusations of a cover up in this case, the spokesman going so far as to say parents thank them for telling them how their sons died.

Bullying culture

Ahn Mi-ja is not one of those parents.

Describing how she felt when she saw her son's alleged attackers, she said: "My skin shook and everything went dark. I couldn't control my mind and I couldn't move when I saw them ... I wanted to do to them exactly what they did to my son."

For Ahn, the most important thing is that her son did not die in vain; that a bullying culture within the military will change -- a culture the military claims starts at school.

"It is pure evil what they did," said Ahn. "A human shouldn't be able to do that. I want a just punishment... I know you can't say something like this will never happen again, but even if it helps a little bit, I hope they get serious punishment."

Six soldiers are currently on military trial, four have been charged with murder.