KEY POINTS:
As British investigators follow the murky trail linking the recent death of former Russian spy, Alexander Litvinenko to the deadly poison polonium-210, renewed international scrutiny is being focused on the black market trade in radioactive materials.
In the past few years, this illicit commerce has been increasing as profiteers and criminals see new opportunities to make money.
Intelligence and law enforcement officials as well as the International Atomic Energy Agency, are concerned that if the trade grows it may result in terrorists obtaining material to make a radioactive dispersal device, or "dirty" bomb - one that uses conventional explosives, like dynamite, to spread radioactive contaminants.
In 1998, al Qaeda's leader Osama bin Laden issued a statement titled The Nuclear Bomb of Islam. In it, he declared it is "the duty of Muslims to prepare as much force as possible to terrorise the enemies of God".
Making or stealing a nuclear weapon would be a difficult undertaking for even the best-organised and well-financed terrorist group. But putting together a radiological bomb is well within the capabilities of al Qaeda and other terrorist groups.
Dhiren Barot, an al Qaeda-linked terrorist who grew up in north London, was jailed for life by a British court last month for his involvement in plans to kill thousands of people in Britain and the United States, possibly by "dirty" bomb.
Unlike a nuclear bomb, such a device is not a weapon of mass destruction.
It would be designed as a weapon of mass disruption - to panic the public and to halt normal economic activity for months, if not years.
Polonium is one of the rarest of some 200 radioactive elements, known as radioisotopes, that are used on a regular basis.
It is as difficult to obtain as it can be to detect, which is why suspicion about the origin of the toxin thought to have caused Litvinenko's death fell, perhaps unfairly, on a Government-connected nuclear research facility in Russia.
However, many other radioisotopes have important civil applications.
These are scattered in thousands of industrial, medical, university and agricultural institutions. They are used for medical therapy, smoke detection and oil well measurements; to sterilise food and equipment; and to power communication devices and navigation beacons.
Only a few are ideal for radiological terror. They include cobalt-60, strontium-90, caesium-137, iridium-192, radium-226, plutonium-238, americium-241, and californium-252.
Unfortunately, the substances most likely to cause greatest harm to humans are also the ones that have significant commercial applications.
The International Atomic Energy Agency (IAEA) says millions of radioactive sources have been distributed worldwide over the past 50 years, with hundreds of thousands currently being used, stored and produced.
In the US, the Nuclear Regulatory Commission reports that about 157,000 licences have been issued for civilian use of radioactive materials. At least 1.8 million devices containing radioisotopes have been licensed in the US.
Since the IAEA started keeping records in 1993, it has had over 820 confirmed cases of unauthorised use reported by member states.
Of the confirmed cases, 560 involved radioisotopes. The trade has been on the rise since 2003 and the IAEA says most confirmed cases have a criminal dimension, even if they were not for known terrorist purposes.
Russia and the US have been co-operating to prevent nuclear and radiological terrorism. In August the US National Nuclear Security Agency announced it had collaborated with Russia to remove 5500 curies of cobalt-60 and caesium-137 from a war-damaged petrochemical production site in Chechnya - enough for at least five "dirty" bombs.
The NNSA added that in three years it had worked with Russian authorities to recover enough radioactive material to build over 200 radiological dispersion devices, or RDDs.
One of the biggest sources of potential RDD material in Russia are radioisotope thermo-electrical generators (RTGs). These were built in the Soviet era to power lighthouses, naval navigation aids, some military facilities and meteorological stations.
Three years ago, there were nearly 1000 RTGs in Russia, with 829 in operation and 169 in storage. Thieves have targeted them on several occasions.
One was recovered from the Gulf of Finland in March 2003. The thieves removed about 500kg of metal shielding the radioactive core. They dumped the remainder on the ice. The 5kg core, with a surface temperature of up to 400C, quickly melted through the ice and sank.
The radiation from the strontium cylinder measured 40,000 curies. Radioactivity 20cm from the unshielded core was enough to deliver a fatal dose within minutes.
The fuel for nuclear reactors that generate electricity typically consists of uranium enriched to a low level, or a mixture of uranium and plutonium.
Most reactor fuel cannot be used to make a nuclear weapon unless one element, uranium-235 is further enriched or concentrated, or unless plutonium-238 is separated from the spent fuel.
However, spent fuel - whether from a power reactor or a research reactor - is extremely radioactive and could be used as part of a "dirty" bomb.
The IAEA says that hundreds of radioactive sources around the world each year are "orphaned" - meaning, that they are lost, stolen or otherwise unaccounted for.
Moreover, over 130 research reactors around the world use fuel concentrated to a level where it could be used in nuclear bombs to produce a devastating explosive chain reaction.
Most of this highly enriched research reactor fuel was provided by Russia and the US.
They are working together to take it back and provide a safe form of fuel that cannot be used to make arms. But this programme will not be completed before 2013.
Meanwhile, the regulations to control and account for radioisotopes that are used in civilian radioactive sources vary from country to country.
Commercial trade in radioactive sources, both within and between countries, is often poorly regulated.
This prompted two specialists to warn in a study published by the National Defence University in the US that the most likely route for terrorists to get 100-10,000 curies of radioactive material was by legal purchase from a legitimate supplier.
