A "dirty bomb" is the type of weapon you would build if you could not construct a conventional nuclear device.
By Dr David Whitehouse
BBC News Online science editor
It would be messy but effective for many reasons.
It would consist of a bomb made of conventional explosives such as TNT, salted with radioactive material.
A dirty bomb could wreak havoc in a big city and cause national panic
Such a bomb would be straightforward to construct if terrorists had access to radioactive material and were able to transport it without detection.
The obvious place to obtain radioactive material would be from a nuclear weapon. It is not unknown for criminals to offer such material.
But to most experts' knowledge, no such dirty bomb has actually been built.
Difficult to counter
Experts say that such a weapon could be locally devastating as well as causing fear and panic nationally.
The dispersal of radioactive material in an urban area could have severe consequences for anyone who was contaminated with radioactive material, though depending upon how much nuclear material was present the conventional explosive could cause the most damage.
Effective decontamination would present health authorities with severe problems, not least because the extent of the nuclear fall-out would at first be unknown.
The US seized Jose Padilla on dirty-bomb related suspicions in 2002
The 1995 Sarin gas attack on the Tokyo subway demonstrated just how difficult it can be to deal with such events. A dirty bomb would be far more difficult to counter.
A contaminated region could be rendered uninhabitable for years and people exposed to radiation would have to be monitored for the rest of their lives. Those contaminated could be subjected to an increased risk of cancer and other diseases.
In the past decade, there have been over 175 cases of terrorist and criminal incidents involving the smuggling and attempted procurement of radioactive material. It has been speculated that some nuclear material was lost after the chaotic fragmentation of the Soviet Union.
In the wake of the Cold War, for example, large numbers of small, portable nuclear generators were simply abandoned in former Soviet territory.
Late in 2001, three woodcutters in northwest Georgia suffered massive injuries after stumbling on one of these radio thermal generators in the forest.
Special teams would be needed to deal with contamination
Alternatively, nuclear material could be stolen from a hospital, a food irradiation plant or some other research site. A recent official study said that US business and research facilities had simply lost track of nearly 1,500 pieces of equipment with radioactive parts since 1996.
In January 2002 it was reported that in the wake of last September's terrorist attacks, President Bush put the US Nuclear Energy Support Team
(Nest) on standby with orders to covertly search for any evidence that a dirty bomb attack could be being planned.
It has been reported that Nest surveillance teams have been patrolling some cities with unmarked vans containing gamma-ray and neutron detectors (both signatures of radioactive materials) to determine if such a terrorist cell planning such an attack could be detected.
Between its establishment in 1975 and the summer of 2002, Nest responded to over 100 cases of nuclear threats from criminals and terrorists. They were all hoaxes.