The execution of Shoko Asahara, the former leader of the Japanese doomsday cult Aum Shinrikyo, is a reminder of one of the most unsettling chapters in modern terrorism. Asahara, who was hanged last Friday alongside six other cult members, was convicted of directing and organising Japan’s worst terror attack—the 1995 sarin nerve gas attack on Tokyo’s subway system.

Aum Shinrikyo members have also been tied to a range of other terror incidents and murders in Japan, including small-scale chemical and biological attacks. The cult was the first terror group to successfully use chemical weapons, and its ability to independently make large quantities of toxins demonstrated that producing weapons of mass destruction (WMDs) isn’t beyond the capabilities of non-state actors. The cult’s legacy is especially relevant today, with the use of chemical weapons on the rise.

Asahara, born Chizuo Matsumoto, started Aum Shinrikyo as a yoga school in the mid-1980s and rode a wave of new religious revival in Japan, quickly recruiting thousands of members. The group mixed Buddhist and Hindu teachings with Christian apocalyptic writings and modern science fiction like Isaac Asimov’s Foundation series. Among its members were top students in technical and scientific fields, including chemical and organic scientists. They donated much of their wealth, time and expertise to the cult and its increasingly bizarre objectives.

Aum Shinrikyo sought to infiltrate elite Japanese society, targeting major corporations, the government and the military. Those seen as opposing the group were put under surveillance, with wiretaps placed in their homes. This grew into a campaign of intimidation and then violence; a lawyer investigating the group was murdered along with his family in 1989, although police didn’t make the connection until after the subway attack.

Failure to gain influence in elections in 1989 prompted the group to pursue more violent alternatives. In 1990, the cult established its first weapons laboratory, focusing initially on biological weapons. Large amounts of botulinum toxin were produced, and then sprayed from trucks near US military bases, the Imperial Palace, Japan’s National Diet and other locations. Crude production methods, however, meant that the toxin was harmless and the ‘attacks’ in 1990 went unnoticed.

By 1992 the group had switched to researching and producing anthrax, but its scientists were again unable to effectively weaponise the toxin. When anthrax spores were released over a neighbourhood in 1993, some pets and birds died, and locals complained of a noxious smell and strange residue, but no human injuries occurred.

Aum Shinrikyo was more successful in its attempts to develop chemical weapons, which started in earnest in 1993 with the production of sarin and other toxins, including VX. In the same year, the group bought Banjawarn Station, 800 kilometres northeast of Perth, and tested the sarin on sheep. Satisfied with the potency of the toxin, in 1994 the cult sprayed it on the homes of three judges ruling on a case against the cult in the city of Matsumoto. The judges survived, but the attack killed eight and injured hundreds. Again police failed to make the connection to Aum Shinrikyo.

On 20 March 1995, cult members carried liquid sarin onto the Tokyo subway, leaving punctured bags of the toxin on busy trains. Thirteen people died and over 5,000 were admitted to hospital, though serious injuries were limited. The failure of the gas to cause the catastrophic effects the cult desired has been blamed on the unsophisticated dispersal method.

Following the attacks, Aum Shinrikyo’s compounds were raided by police, revealing disturbingly large stocks of chemical and biological toxins and the means to manufacture more. Raids also revealed the extent of the cult’s operations, including involvement in the Matsumoto attack, and other assassinations and attacks.

The most alarming lesson to be taken from Aum Shinrikyo’s campaign is not the way the cult used WMDs, but that it didn’t have to steal them and was able to produce the toxins independently. With perhaps only a little more time or luck in its research, the cult could have killed tens of thousands across Japan.

Prior to the 9/11 attacks, counterterrorism experts were sceptical that terror groups had either the ability or the desire to independently develop WMDs. RAND expert Brian Jenkins’ 1975 maxim that ‘terrorists want a lot of people watching, not a lot of people dead’ was the prevailing view. Aum Shinrikyo’s attacks called that view into question, but there was still scepticism about the capability of terror groups to develop WMDs without a state sponsor.

Modern terrorists have proved that they are willing to kill on a large scale, and that they will seek to acquire or develop WMDs to do so. Aum Shinrikyo’s toxic legacy and the threat of such an attack will only grow as technologies and expertise in chemistry, medicine and biology become more globalised in the 21st century.