Seiichi Endo was born on 5 June 1960 in Sapporo, Hokkaido, Japan. Raised in northern Japan, he was described as quiet and academically gifted. After completing secondary school, he pursued higher education in science, earning a degree in veterinary medicine. His studies included virology and genetic engineering, and he later advanced to graduate work at Kyoto University, one of Japan’s most prestigious institutions.

During this period, Endo developed an interest in biology and toxicology, skills that would tragically be co-opted by the cult movement he later joined. Despite a promising academic career path, he became involved with the Aum Shinrikyo cult in the late 1980s, following its charismatic leader Shoko Asahara. Asahara’s mix of pseudo-Buddhism, apocalyptic prophecy, and authoritarian control drew in many young, educated Japanese, particularly in scientific and technical fields. Endo, with his background in virology and veterinary sciences, was quickly recruited into Aum’s chemical weapons program.

By the early 1990s, Aum Shinrikyo had grown into a powerful and dangerous organization, combining elements of religion, science, and terrorism. Its leader, Shoko Asahara, preached apocalyptic visions of an impending war between the cult and the Japanese government. To prepare, Aum stockpiled weapons and began developing chemical and biological weapons.

Seiichi Endo, under Asahara’s orders, became the primary figure responsible for producing sarin gas, a deadly nerve agent that causes convulsions, respiratory failure, and death in even small doses. Endo’s laboratory successfully synthesized sarin on a scale that could be weaponized, marking a turning point in Aum’s capacity for mass violence.

Before the Tokyo subway attack, Aum had already used sarin in smaller-scale operations. In June 1994, the cult released sarin in Matsumoto, Nagano Prefecture, killing 8 people and injuring over 500. Endo’s production work enabled this deadly strike, although at the time responsibility was not fully attributed to the cult.

The most infamous attack came on 20 March 1995, when five Aum members carried liquid sarin in plastic bags and punctured them on crowded subway trains converging on central Tokyo. The coordinated attack killed 13 people and injured over 6,000, many suffering from long-term neurological and respiratory damage. Though Endo did not personally release the gas, his role as the chief producer of the weapon was central.

Following the attack, Japanese police launched massive raids on Aum facilities. Endo was arrested in May 1995, along with other senior cult figures. During his trial, prosecutors argued that his scientific knowledge and ability to mass-produce sarin made the attack possible. Endo was convicted of murder, attempted murder, and violating laws on chemical weapons.

After years of appeals, his death sentence was finalized in 2002. On 6 July 2018, he was executed by hanging at the Tokyo Detention House, alongside six other senior Aum Shinrikyo members, including Shoko Asahara.