Tokyo, 20 March 1995, morning rush-hour.
Millions of commuters step out into a bright spring morning and on to one of the world's busiest underground systems.
Also on board the trains: five bags filled with liquid nerve agent, left by members of a doomsday cult.
The packages were leaking. Passengers felt stinging fumes hitting their eyes.
The toxin struck victims down in a matter of seconds, leaving them choking and vomiting, some blinded and paralysed. Thirteen people died and at least 5,800 were injured in five co-ordinated attacks on three train lines.
The cause was Sarin, a nerve agent developed by the Nazis. It was the worst domestic terror attack ever carried out on Japanese soil.
The culprits were Aum Shinrikyo, an obscure religious group who believed the end of the world was coming.
After years on death row, the cult's leader Shoko Asahara was put to death on 6 July, along with several of his followers.
Where did the cult come from?
Aum Shinrikyo, whose name means "supreme truth", began in the 1980s as a spiritual group mixing Hindu and Buddhist beliefs, later working in elements of apocalyptic Christian prophesies. The group's founder, Shoko Asahara, declared himself to be both Christ and the first "enlightened one" since Buddha.
The group, whose name is often shortened to just Aum, gained official status as a religious organisation in Japan in 1989. Asahara picked up a sizeable global following, speaking at universities and writing books. At its peak, Aum had tens of thousands of members worldwide.
Former cult members have testified that they paid handsomely for rituals involving Asahara's hair and bathwater - despite the group urging followers to reject materialism. One described paying more than £6,115 ($8,100) in 1988 for a "blood initiation" where he drank what was said to be the leader's blood.
Many of Asahara's followers in Japan were students at elite universities. They were young people from academically pressured backgrounds who had similarly pressured careers ahead of them - and the cult promised them a more meaningful life.
Aum gradually became a paranoid doomsday cult, convinced the world was about to collapse into World War Three and only its members would survive.
It became increasingly violent, kidnapping, injuring and killing opponents, and even using chemical and biological agents in other attacks.
What happened after the attack?
In the months after the March 1995 attack, the group made several failed attempts to release hydrogen cyanide in various stations.
The subway atrocity shocked Japan, a country that prides itself on low crime rates and social cohesion. It also raised questions about police failings to investigate previous allegations of criminal activity by the group.
Japanese novelist Haruki Murakami wrote a non-fiction book about the incident titled Underground: The Tokyo Gas Attack and the Japanese Psyche, and interviewed 60 victims.
Scores of Aum members have faced trial over the attack, and 13 were sentenced to death, including Asahara.
The last to be tried in connection with the subway attack was Katsuya Takahashi, arrested in June 2012 after being on the run for 17 years. He was sentenced to life in jail.
Cult member Makoto Hirata was also jailed in 2014 for the abduction of a 68-year-old man and his involvement in two bomb attacks.
Is the cult still active?
The group went underground after the Tokyo attack, but did not disappear, eventually renaming itself Aleph.
Another smaller group, Hikari no Wa (Circle of Rainbow Light), headed by Aum's former spokesman and Asahara's successor Fumihiro Joyu, was formed in 2007.
Joyu claimed to have distanced his group from worship of Asahara.
Aum Shinrikyo is designated as a terrorist organisation in the US and many other countries, but Aleph and Hikari no Wa are both legal in Japan, albeit designated as "dangerous religions" subject to heightened surveillance.
Some estimates say they have 1,500 followers between them, with reports the number is slowly growing.
Did Aum operate outside Japan?
The group operated in former Soviet states after the collapse of the USSR.
In March 2016, Montenegro expelled 58 foreigners suspected of association with Aum Shinrikyo. They were gathered at a hotel they had rented in the former Yugoslav region.
Four were from Japan, the interior ministry said, but 43 were from Russia, seven from Belarus, three from Ukraine, and one from Uzbekistan.
Russian raids in April 2016 targeted 25 properties. The group is illegal in Russia, but prosecutors said at the time it may have up to 30,000 Russian followers.
A few human rights advocates in Japan have defended Aum cultists who have not been found guilty of crimes, saying they were unaware of plans to attack the Tokyo subway.
But members have little prospect of social or official acceptance, despite attempts to distance themselves from the events of more than two decades ago.