In all, 13 people died and 6,300 were sickened after the Aum Supreme Truth cult released sarin in five subway trains during co-ordinated rush-hour attacks on March 20, 1995.
Thirteen Aum members, including cult leader Shoko Asahara, remain on death row, while others are serving prison sentences. The last fugitive was arrested in 2012.
In what some believe was an attempt to divert the authorities that Asahara thought were closing in on his base in the foothills of Mount Fuji, he sent five teams of two people to attack the Tokyo subway.
Five adherents-among them a senior medical doctor and several physicists-dumped packages of sarin on busy trains, puncturing them with sharpened umbrella tips, before being driven away from a pre-determined station by their co-conspirators.
The nerve gas, so toxic that a single drop can kill a person, evaporated over the following minutes as thousands of unwitting commuters got on and off each train.
Staff and passengers were among the dead. Many of those sickened only realised what had happened as their symptoms worsened throughout the day and news broadcasts began piecing events together.