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 £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.

Disturbing.
But a good reminder of who we are dealing with.
FYI, the article link is not working.
The article has been moved here:
http://www.thisiscornwall.co.uk/displayNode.jsp?nodeId=115860&command=displayContent&sourceNode=115858&contentPK=9503771
I think he was joking. But thank god they found them. I just wish the Brits would wake up.
It is beyound my imagination why the british government is protecting these individuals,while they are trying to inflict such horrible sufferings on the country which has has been and it still is,a gracious and generous host??!!
These muslim fanatics are welcomed to Britain,they are housed and fed,provided with access to education,financial and medical assistance,there are laws that protect them,there is political corectness that is making sure nobody is offending them...and yet they want more,they want the whole country,they want to subjugate the whole nation and turn the island into the first musilm country in Europe.
Why on earth are they still allowed to enter Britain,why are they still allowed to build mosques,why aren`t they deported and never permitted to go back?!
Until the British recognise that they are at war they will do little to defend themselves.
WHy do the media play down the horrific massacres in third world countries by Islamic killers ? When filpinos, Nigerians, Sudanese, indonesians are butchered we hear almost nothing.
England is only one of many targets.
The huge infux of Islamic immigration has created a nightmare situation, a frightening trojan horse reality in which the enemy will attack from within. No EU country with a large Muslim percentage is safe.
Now they have proved that 500 kilos of explosives can change a government overnight, their resolve will be further strenghtened.
Democracy has infinitely improved our lives but it has also exposed our weaknesses in dealing with those who wish us destroyed.
"I just wish the Brits would wake up."
If we'd been asleep we wouldnt have foiled this plot.
Until the British recognise that they are at war they will do little to defend themselves.
If we hadn't recognised we were at war, we wouldn't have foiled this plot.
"Why on earth are they still allowed to enter *Britain*,why are they still allowed to build mosques,why aren`t they deported and never permitted to go back?!"
* = enter any other western nation in to this slot.
Thank God for the wonderful job our MI5 & MI6 agencys did in stopping this before they unleashed hell.
And as a Brit, if they are found to be immigrants, I hope they do deport them. I shall certainly be one of the first to write to parliament urging the government to do so.
The entire world's annual production of Osmium is less than 500 kilograms a year - it is rarer than platinum.
There are plenty of deadlier and cheaper chemicals available.
Terrorists may be evil, but they are not stupid.
Even the shoddy original ABCnews story does not claim that anybody actually succeeded in taking delivery of, or stealing, any Osmium Tetroxide whatsover, let alone making it into a "chemical weapon".
If it is true that "intelligence sources" leaked details of a GCHQ or MI6 operation to ABCnews, then *that* is the most serious part of this "story".
http://www.spy.org.uk/spyblog/archives/000251.html
I know this isn't the core topic, but isn't it interesting their are still Oppenheimers in the bomb expertise business.
It sounds (unless all the newspapers have been deliberately misbriefed) that osmium is only useful as a chemical catalyst and so would enhance an explosion, but not greatly. It would also make it more complex to construct the bomb.
Maybe someone in MI6 (OR MI7 8 or 9 the really secret ones) put out a fake anarchists handbook, including in the bomb recipes substances which are
a) useless
b) very expensive
c) really easy to trace as they are only sold in small quantities to very specific businesses.
Good grief Adela!
I don't know which country you are from (or which planet)but what you are suggesting is nothing less than what Adolf Hitler was in deep pursuit of. Ethnic cleansing.
It seems that the answer you are suggesting is a total sweep of society to find out who is a Muslim and 'dispose of them'. Her Majesty's government and MI6 and intelligence were monitoring this group since February looking for more proof and also the potential that more would be uncovered.
I would rather trust our government than you, a thousand times over.
Every refugee or asylum seeker is treated with some measure of dignity and respect - we do have cultural differences and let me say this; the IRA have slaughtered more people in the UK than can be believed. George Bushes people have shot and killed thousands of their own people in the USA whilst arguing for the right to carry firearms.
We can't even sort out the stuff that is happening on our own doorsteps but to you the answer is so easy. Deport them. Shame on you.
The Muslim community in the UK has given us far less problems than the Irish Republican Assassins (Army).
At least the Muslims have given us corner shops.
What have you contributed?
Exactly.
I refer you to the answer that KC gave:
Thank God for the wonderful job our MI5 & MI6 agencys did in stopping this before they unleashed hell.
And I also agree, if they find out they were immigrants they should deport them. After we have made them watch 4 hours of cricket.
osmium tetroxide in 0.1% N solution is used in scanning electron microscopy in order to fixate tissues so that they can then be dehydrated and coated with gold-palladium which then allows for the scanning electron beam to bounce off the topography of the tissue for data collection that is then projected through a cathode ray tube. a liter of this solution used to cost $400/gram about 15 years ago. in order for it to be usable as a killing agent, it would have to be delivered in its liquid form or be atomized like dust.
OsO4 must be interacted with in a fume hood so the scientist does not breathe it and subsequently have his lungs fixated leading to irreversable damage that causes death by asphyxiation within a minute or so. it would not be very effective in the open air, and for full impact would have to be released in a room.
still, it is frightening to see terrorists taking an interest in such an exotic compound.
incidentally, mustapha, the murder rate in the u.s. has dropped in the past decade from 15 to 8 per 100,000. england and the rest of western europe have risen from an average of about 2.5 to 5 per 100,000 in the same time frame. the europeans are now studying american crime-fighting behaviour in order to improve your own and head off this alarming trend. murderers in the u.s. are not "george bush's people" as you have stated. in fact, it is the democrats who promote the rights of felons. a high rate of gun ownership by good citizens typically makes the crime rate go down.
correction to price of 0.1% N solution of OsO4: it used to cost $400/liter 15 years ago. not sure what the price is today.
Fair point Ted - it's not what I was meaning, I apologise, it came out wrong.
I still stand by my view about cricket.
further correction: it's 1% N OsO4 solution. also used in transmission electron microscopy specimen preparation (my, how the memory fades).
incidentally, mustapha, i do appreciate your statement about cricket. i watched a match for about two batters once and could not for the life of me see how it could possibly be as interesting as baseball. perhaps we could punish terrorist prisoners by showing them some of our reality t.v. shows, too.