Five Aum Shinrikyo members boarded subway cars on three different lines in central Tokyo during rush hour, carrying plastic bags filled with sarin.
"We should have heard more stories from the people involved and deepened discussions in society to get to the bottom as to why such crimes related to religion happened", Ogawara said.
Japanese doomsday cult leader Shoko Asahara had been on death row for masterminding the 1995 deadly Tokyo subway gassing and other crimes.
In this 2015 photo, Tokyo subway workers offered silent prayer 20 years after the poison gas attack.
Cult leader Shoko Asahara was executed alongside six of his followers.
"With the execution, I feel that the opportunity to discover (why) has been lost", Moriyama said.
The convicted also assaulted and murdered wayward followers and people who helped members leave the cult.
Aum Shinrikyo was founded by Shoko Asahara of Japan in 1987 and sprang into global focus in 1995, when its adepts sprayed the nerve agent Sarin in the Tokyo subway. The initial death sentence in 2004 became final after his defense team could not file an appeal citing his mental state.
A riot police officer stands guard outside the Aum Shinrikyo cult headquarters 6th compound as the raids continued on 11 May 1995, searching for the mastermind behind the attacks.
Inoue headed Aum's intelligence unit and was one of the few cult members who later turned against Asahara.
Chizuo Matsumoto, the cult's leader who went by the name Shoko Asahara, was the first to be hanged, media said as it broke into regular programming to report the news.
It renamed itself Aleph in 2000 and two splinter groups have been formed, including one established by high-profile former member Fumihiro Joyu. The sarin gas attack the cult carried out in Tokyo shattered Japan's sense of public safety.
At a time when the global trend is toward abolishing capital punishment, Japan's death penalty system has sparked worldwide criticism, especially over the secrecy surrounding its executions, and prompted critics to push for its abolition.
Asahara pleaded not guilty and never testified, only muttering and making incoherent remarks in court during the eight years of his trial, according to Reuters. Asahara also preached apocalyptic prophesies, which were the genesis of the subway attack: He believed the end of the world was coming, and the attack was a way to prepare.The Asahi Shimbun/The Asahi Shimbun via Getty ImagesPRESS CONFERENCEShoko Asahara (center) at a press conference in October 25, 1990.
Over the years, the group managed to lure in followers from some of Japan's top universities and boasted some 10,000 followers in Japan and another 30,000 in Russian Federation.