The possibility of a rogue state, terrorist group, or individual acquiring and using chemical or biological weapons is a real treat. The last successful biological attack in the US happened in 1984 when a cult by the name of Rajneeshee in The Dalles Oregon used Salmonella to poison several salad bars of local restaurants. The purpose of this biological attack was to influence local elections which were taken place. Authorities did not find out about the deliberate biological attack until a year later while investigating another unrelated case. Following the September 11 attacks in New York several letters were mailed to media and politician personal with Anthrax, biological agents are a very effective way to spread terror and panic amongst a group of people or a targeted city. Biological agents are easily obtainable, cheap to make, very lethal if employed correctly and extremely hard to detect.
Chemical agents are another deadly tool witch terrorist groups, states or individuals can use to created panic, mass hysteria, and high casualty rate. Chemical agents are extremely difficult to handle, produce, and the means of delivery can often be extremely complex. The special handling of chemical agent makes it extremely difficult to transport to the intended target, equipment is available and emplace to monitored and detect any potential chemical attack or agents which makes it that much difficult to get the chemical agent across international borders.
The treat of a chemical attack is as I mention before a very real threat thought the world, we can see that threat in countries such as Syria, Iran, parts of Iraq control by ISISL, North Korea, and Pakistan just to mention a few . Syria is the latest country to have been known to use chemical weapon on its own people during the fighting with Syrian revels. Syria was being backed up by Russia and yet after it was discovered to have employed chemical weapons which is a violation of international law no sanctions or repercussions were given to the Assad regime of Syria. In my opinion chemical weapons pose a more likely treat to be use by establish nation against nation (eg Pakistan against India) versus a rogue state such as Hezbollah or Al-Qaida. Biological weapons I see as a more likely attack by a rogue state or individual group or person. The reason it is a most likely to be use in a terrorist attack is because biological agents are easy to acquired, the agents used to create a biological agent can be grown or acquired through 3rd party groups or the black market. The employment of biological weapons can be a very complex, requiring expensive delivery weapons an equipment , but biological weapons can also be deployed in a less complicated and cheaper means such as was the case in The Dalles,Oregon in 1984 as I mention before. A bio warfare laboratory could easily be build and utilize to create biological agents with limited training and knowledge by the operators of this lab. The equipment needed to build and created an operational bio warfare laboratory are widely available through commercial means, this would mean unwanted attention and suspicious would be avoided by the terrorist group building the laboratory. Agro terrorism is somewhat of a significant threat, agro terrorism would require a development of pathogens to affect agricultural food supplies and animals. The development, production, and storage of these pathogens would require expensive sophisticated equipment and facilities. It is certainly a possibility but a not likely first choice of attack for a terrorist group.
The more likely use of weapons of mass destruction by a group of terrorist would be biological, the reason for been biological is because of how easy it is to obtain the compounds use to create a biological agent. The means of delivery of a biological agent are extremely easy and as I mention before there is no equipment to monitor of detect a biological attacks. Something as simple as poisoning a water supply or food supply