Many of the inmates sentenced to death over a deadly sarin gas attack on the Japanese subway appear to be one step closer to their execution dates.

Key points: The sarin gas attack in Tokyo was the first act of terrorism using a weapon of mass destruction

The sarin gas attack in Tokyo was the first act of terrorism using a weapon of mass destruction Thirteen members of the Aum Shinrikyo cult are on death row

Thirteen members of the Aum Shinrikyo cult are on death row The Japan Society for Cult Prevention and Recovery believes we can learn from convicted cult members

But some victims believe most of the convicted Aum Shinrikyo cult members should have their lives spared, fearing they could become martyrs if they are executed.

Thirteen people died and more than 4,000 were injured 23 years ago today, when the doomsday cult carried out its deadly chemical weapons attack on the world's busiest metro system.

The cult had prepared for the attack at a Banjawarn station, a remote sheep station it owned in Western Australia.

Seven of the 13 inmates on death row have been transferred from the Tokyo detention centre to other facilities around the country — prompting speculation the Government is preparing to execute them.

Hiroyuki Nagaoka has dedicated his life to helping Aum Shinrikyo members escape the doomsday cult.

"I met [cult leader] Shoko Asahara 29 years ago — and I had a strong intuition that if we allow this group to go on they would do something very terrible," he said.

He helped his son escape the cult, which made him a target.

Aum members tried to assassinate him with VX nerve agent in January 1995 — they failed — but two months later the cult carried out its deadly attacks on the Tokyo subway.

Hiroyuki Nagaoka says Aum Shinrikyo recruited members with long sermons. ( ABC News: Jake Sturmer )

Subway sarin attack

Thirteen people died, 50 people were left permanently injured, and thousands of others were temporarily blinded by the gas in the 1995 attack.

Five men released the deadly nerve agent in carriages of crowded commuter trains during Tokyo's morning rush hour.

The sarin gas attack in Tokyo is regarded as the first ever use of a weapon of mass destruction in an act of terrorism.

The perpetrators of the crime thought they were carrying out a holy act in line with the beliefs of the doomsday cult.

The prosecutors of the criminal trials believe that day was chosen to divert the attention of police who were planning a raid on the cult's headquarters.

Asahara was found guilty of masterminding the attacks in 2004 and sentenced to death by hanging, but his execution was postponed while the appeals of his fellow criminals were heard.

He is now 63 years old, and spends his days in solitary confinement.

The appeal pathways for his fellow criminals were exhausted in January.

The head of the cult, Shoko Asahara, was found guilty of masterminding the attacks in 2004. ( Reuters )

'Much to learn' from death row inmates

Because the inmates are on death row, contact with them is severely limited.

Social psychologist Kimiaki Nishida performed psychological evaluations on many of the convicted cultists.

Little is understood about the motivations of Aum Shinrikyo, and he said there would be a huge opportunity to learn from them if their lives were spared.

"The question that I have is how, what triggered that change and how did their minds change?" he said.

"How did this transformation occur? And if I could find that out, it would be a great help in countering these various terrorist groups."

Kimiaki Nishida is hoping to understand why people join the Amu Shinrikyo cult. ( ABC News: Jake Sturmer )

He is chair of the Japan Society for Cult Prevention and Recovery, which wants 12 of the 13 convicted cultists to have their sentences commuted.

The society said it did not have a position on the execution of Asahara.

Mr Nagaoka is also a member of the society — and said Aum Shinrikyo managed to convince its victims with exceptionally long sermons that started early and finished late.

"In such conditions you lose your ability to think," he said.

They were brainwashed to think it was wonderful to kill people because it would help them and send their souls to heaven.

"They really believed — when I do go and meet them in detention centres they all say I want to die," he said.

"I always tell them don't be absurd, you have to tell your experience so the other members can return to normal beings who can think with their own heads."

Any decisions on clemency are up to the National Government's Cabinet.

Executions in Japan are cloaked in secrecy so it is impossible to predict exactly when any of the cult members will be hanged.