The attack was foiled because the suspected terrorists were being monitored. From This Is Cornwall:
Police and the security services have foiled a terrorist plot to launch a poison gas attack in Britain.
A plan to use a highly toxic chemical called osmium tetroxide was uncovered after communications between terror suspects were monitored.
The chemical compound, which can be bought on the internet for Â£17 a gram, causes victims to choke to death in agony and turns skin black. It also attacks the cornea and can lead to blindness.
Anti-terror chiefs fear the target for an attack could have been the London Underground system, Gatwick airport or a crowded public area like a shopping centre.
Experts say the chemical could have been “piggy-backed” on to a conventional bomb which would see the invisible fumes dispersed into a crowd of people.
As they died victims would have suffered asthma-like symptoms known as “dryland drowning”.
Suspects’ conversations were eavesdropped on at the GCHQ electronic listening centre and police moved to disrupt the alleged plot at an early stage before any osmium tetroxide was obtained.
While al Qaida has included plans for chemical attacks in training manuals, it has so far used conventional devices and the use of poison gas would have marked a new departure in its strategy for causing chaos.
Andy Oppenheimer, a nuclear, biological and chemical weapons expert for Jane’s Information Group, said osmium tetroxide was an unusual choice as a chemical weapon but it could kill.
He said if terrorists were going to use it they would be likely to do so in a small bomb in a confined space.