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Hydrogen Cyanide attack in New York

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12/04/2006 04:59 AM
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Hydrogen Cyanide attack in New York
The Al-Qaeda leadership called off a hydrogen cyanide gas attack on the New York subway system in 2003, according to a recently published book that estimated the attack would have resulted in 3,000 casualties.

This estimate is probably exaggerated. A review of a jihadist manual for the construction of a mubtakkar gas device similar to the one described found it lacking in detail and unlikely to be an effective weapon.

While it may never be known why Al-Qaeda called off the attack, it is possible that the leadership were aware of the limitations of the weapon and preferred to wait until a more effective chemical weapon had been developed or acquired.

The recent revelation that Al-Qaeda called off a chemical weapon attack on New York's subway system has refocused attention on the threat from terrorist weapons of mass destruction (WMD).

However, hydrogen cyanide (HCN) gas - the weapon of choice in the alleged plot - has significant limitations. While the ingredients are widely available, it is not easy to effectively weaponise the gas.

The New York plot

Investigative journalist Ron Suskind claims in his new book, The one percent doctrine, that in 2003, a joint Saudi-US counter-terrorist unit found instructions for making a mubtakkar (invention in Arabic) device that releases deadly hydrogen cyanide (HCN) gas on a suspect's computer.

He describes the device as a "fearful thing" consisting of a "canister with two interior containers: sodium cyanide is in one; and a hydrogen product, such as hydrochloric acid, in the other; and a fuse breaks the seal between them". The HCN that is released has only a faint smell of peach kernels or almonds and can be lethal if inhaled.

Suskind wrote: "In the world of terrorist weaponry, this was the equivalent of splitting the atom."

The book claims that an Al-Qaeda insider told the CIA that Yusuf al-Ayiri, a leading Al-Qaeda operative in Saudi Arabia, told senior Al-Qaeda leader Ayman al-Zawahiri about a plot to attack New York's subway system during a meeting in January 2003. Ayiri said a group of jihadists had infiltrated the US via North Africa in 2002 and thoroughly reconnoitred the target. An attack involving several mubtakkar was only 45 days away, but Zawahiri called off the operation. Ayiri passed the message on to the plotters, who recognised the Al-Qaeda leadership's authority even though they were a self-activated cell that had been operating independently.

Suskind states that the CIA officials who briefed the US government did not know why Zawahiri cancelled the attack, although he quotes President George W Bush as asking at the time: "Is it because he did not feel this was sufficient for a 'second wave'?"

Suskind then lists the damage the HCN attack would cause: "Ten subway cars at rush hour - 200 people in a car - another thousand trampled in the underground in a rush-hour panic."

On the basis of the projected 3,000 casualties, Suskind speculates that Zawahiri was planning something far larger to follow the 11 September 2001 attacks. However, I believe that the author may be over-estimating the effectiveness of the mubtakkar being developed by jihadists.

Jihadist weapons research

Jihadists have long attempted to develop cyanide-based weapons. The group behind the bombing of the World Trade Center in 1993 included sodium cyanide in their truck bomb, apparently with the intention of creating a HCN gas cloud that would kill people who survived the primary explosion.

The judge who sentenced four men involved in the bombing said: "Thank God the sodium cyanide burned instead of vaporising. If the sodium cyanide had vaporised, the cyanide gas would have been sucked into the north tower and everybody in the north tower would have been killed."

Ahmed Ressam, who was arrested crossing into the US from Canada with explosive materials in 1999 and convicted of conspiring to attack Los Angeles International Airport, confessed that he had been trained in the use of poisons when he attended Al-Qaeda's Darunta training camp in Afghanistan.

The claim that Al-Qaeda was experimenting with chemical weapons was confirmed by videos found in Afghanistan after the US intervention in 2001. In August 2002, CNN aired footage of experiments in which dogs were killed in gas chambers as jihadists talked off-camera in Egyptian accents. It is believed that the gas used in the film was HCN and that Midhat Mursi al-Sayid Umar, an Egyptian known as 'Abu Khabab', oversaw the experiments.

While Abu Khabab was reportedly killed in an air strike in northern Pakistan on 13 January, he is still listed on the FBI's Rewards for Justice website, which offers a bounty of USD5 million for information on his whereabouts. It claims that he has "provided hundreds of mujahideen with hands-on poisons and explosives training [and] proliferated training manuals that contain recipes for crude chemical and biological weapons".

It has been reported in the press that Khamel Bourgass, the Algerian convicted in the UK in connection with an attempt to manufacture the poison ricin, was one of Abu Khabab's students. Bourgass' poison recipe also included instructions for making cyanide. However, ricin's limitations as a mass-casualty weapon suggested that the poisons training provided in Afghan camps was of little value.

According to Duncan Campbell, an investigative journalist tasked by the defence to track the provenance of the poison information, Bourgass' recipe was originally published in the 1980s by a US survivalist.

Another 'chemical lab' was discovered in the Iraqi town of Fallujah after it was cleared of insurgents by US and Iraqi troops in November 2004. They found significant quantities of sodium cyanide and hydrochloric acid, along with other chemicals, and documents suggesting they were experimenting with devices that would produce hydrogen cyanide gas.

The jihadist manual detailing the manufacture of an improvised HCN dispersal device but suggests a lack of sophistication. The document, which was posted on a jihadist website, refers to a mubtakkar that seems similar to the one described by Suskind. The manual claims that it is the product of "months and years of experiments".

HCN limitations

HCN is a 'blood agent' that prevents a victim's cells from absorbing the oxygen carried by the blood. A large concentration of inhaled cyanide can cause hypernea (rapid or deep breathing) in 15 seconds, seizures in 30 seconds; respiration stops five minutes after exposure and death occurs a few minutes later.

The problem with using the gas as a weapon is that it is only lethal in high concentrations and causes few casualties at low concentrations. The concentration needed to kill 50 per cent of people (LCt50) is estimated to be around 2,500 to 5,000 mg-min/m3. The highly effective VX nerve agent has an estimated LCT50 of just 10 mg-min/m3. The gas is lighter than air and highly volatile, meaning that it is an unstable molecule that decomposes quickly, and consequently dissipates soon after release. It is considered, therefore, to be a 'low-end' warfare agent that is easy to manufacture but unlikely to inflict many casualties on a battlefield.

However, HCN is lethal in a confined space and has been used for judicial executions in gas chambers in the US. More notoriously, a form of hydrogen cyanide called Zyklon B was used in the Nazi death camps.

Yet a terrorist would struggle to produce an effective weapon if he worked from the mubtakkar manual. According to Dr Brian Jones, an expert on weapons of mass destruction who reviewed the document, the device would be viable.

However, he said the "details are so generalised that without more instruction there would be a high potential for failure unless the manufacturer conducted quite extensive and probably dangerous trials".

The manual also recommends that ventilation or air-conditioning systems should be used to spread the gas, even though this would make it even harder to produce a lethal concentration. Indeed, the 1993 World Trade Center bombers would have needed an extremely large amount of sodium cyanide to kill anyone in the north tower.

Terrorists would even struggle to kill all the passengers in a subway train carriage with a mubtakkar-type device, according to another expert who has investigated the threat from a terrorist HCN attack and talked to Jane's on the condition of anonymity. While it is technically possible that terrorists could deploy a mubtakkar capable of filling a carriage with a lethal concentration of the gas, the passengers would be alerted to the attack by the explosion of the detonator. While HCN is colourless and fairly odourless, the type of detonator recommended in the mubtakkar manual gives off caustic fumes that would repel the passengers from the device.

The passengers of a subway carriage would, therefore, probably be aware of the attack and activate the emergency alarm. They would stand a better chance of survival if they opened or smashed any available windows, got close to the ground, or moved to another carriage. Even if they were incapacitated, the gas would dissipate rapidly to a non-lethal concentration once the train reached the next station and the doors opened automatically. Large volumes of air flow around subway systems due to the movement of the trains. Even a well-researched mubtakkar that combines laboratory grade reagents, therefore, would cause few fatalities if the train reached a station within a few minutes of initiation.

Aum Shinrikyo

The Aum Shinrikyo (Supreme Truth) cult demonstrated hydrogen cyanide's limitations as a terrorist weapon in the mid-1990s. More famous for the sarin nerve agent attack on the Tokyo subway system that killed 12 people on 20 March 1995, the group also attempted three attacks using HCN, according to the Monterey Institute of International Studies' WMD Terrorism Database.

On 5 May 1995, cult members planted two plastic bags - one containing sulphuric acid, the other sodium cyanide - in a Tokyo subway restroom. They started a fire that gave them time to escape before it ruptured the bags to initiate the chemical reaction. However, the fire alerted the authorities and was extinguished. HCN gas was released, but only injured four people, who complained of throat irritation and respiratory problems.

Modified HCN devices were deployed in Tokyo on 4 and 5 July 1995. They included a small electric motor attached to blades that were intended to rupture the bags when a timer closed a circuit. While the stealthier initiation of the devices should have made them more effective, they still suffered technical problems. The device used in the 4 July attack failed to mix the sulphuric acid with the sodium cyanide and caused no casualties. The 5 July device created HCN, but only caused one casualty, who suffered temporary breathing difficulties.

While Aum Shinrikyo demonstrated that chemical weapons are within the technical reach of terrorists, they also demonstrated the difficulties non-state actors face in weaponising such agents. The cult had vast financial resources that enabled it to recruit organic chemists from Japanese universities and build a sophisticated weapons production facility called 'Satian Seven'. Yet, it only killed 20 people (as well as up to 20 of its own members who were experimented on with sarin) in its attempts to bring about its leader's prophecies.

Zawahiri's call

It is possible that Zawahiri knew that the mubtakkar plot on the US subway was unlikely to cause many casualties and considered it an inadequate follow-up to the 'spectacular' 11 September 2001 aircraft impact attacks. While Al-Qaeda's development of chemical weapons seems to have been more limited by a lack of technical knowledge than Aum Shinrikyo's, Abu Khabab's experiments are likely to have highlighted the shortcomings of primitive, improvised HCN devices.

While Zawahiri may have called off the New York attack because he thought it would be too limited, it must be noted that he considered the 7 July 2005 suicide bombings in London, which killed 52 people, sufficiently damaging to claim responsibility for in Al-Qaeda's name. It seems that more conventional homemade bombs, such as the ones used by the 7 July suicide bombers, are similarly easy to build and more effective in terms of producing fatalities.

Yet, a gas attack would be psychologically more terrifying than conventional bombs and an evacuation could result in more casualties if panic breaks out. The psychological effect of chemical weapons was demonstrated by the 20 March 1995 Aum Shinrikyo sarin attack: hospitals classified the casualties as 17 critical (12 of whom died), 37 severe, 984 moderately ill, and 3,686 psychogenic patients who had no real physical symptoms.

Zawahiri's decision to call off the mubtakkar attack on New York is likely to remain a matter of debate for the foreseeable future. It is possible that Al-Qaeda would prefer to wait until it has a more effective capability before it launches a chemical weapons attack. Yet it has trained its followers - many who now seem to be operating independently - to make crude chemical weapons. It seems strange that none of these footsoldiers have attempted such an attack. At the same time, the arrest of people like Khamel Bourgass has created sensational press headlines that suggest Al-Qaeda has a credible WMD capability. While no strong evidence has been made publicly available to suggest this is the case, the publicity enhances Al-Qaeda's reputation as a powerful force.

Chemical and physical properties
Boiling point 78ºF (25.7ºC)

Vapour pressure 740 mm Hg


Vapour 0.99 at 68ºF (20ºC)

Liquid 0.68 g/ml at 77ºF (25ºC)

Solid NA

Volatility 1.1 x 106 mg/m3 at 77ºF (25ºC)

Appearance and odour

Gas: odour of bitter almonds or peach kernals


In water complete at 77ºF (25ºC)

In other solvents completely miscible in almost all organic solvents

Environmental and biological properties

Detection ICAD; M256A1 kit


In soil 1 hour

On material Low

Skin Water; soap and water decontamination

Biologically effective amount:


(mg min/m3) Ct50: 2,500-5,000 (death)

LCt50: 2,500-5000

(time dependent)

Liquid LD50 (skin): 100 mg/kg

Source: Jane's Chem-Bio Handbook
Time is too long. Space is too big.