Imagine the effect of the recent Boston bombings if instead of ball bearings those pressure cookers contained a nerve agent like Sarin. Fatalities could easily have numbered in the hundreds and local hospitals would have been overrun with casualties. A dirty bomb laced with finely powdered radioactive materials is only slightly more difficult make and would also have devastating results. There is a 2004 made for TV movie called Dirty War that realistically portrays the effects of the detonation by terrorists of a radioactive dirty bomb in central London. Sadly this movie could soon become a reality.
Why bother with Sarin? A competent terrorist could cook up any number of equally lethal agents with off the shelf ingredients. Look at what two barely trained kids managed to pull off with a pressure cooker and some fireworks. Now imagine a couple of dozen serious professional types could do.
No need to imagine. Sarin attacks were carried out by (rogue) scientists in the Tokyo subway in 1995:
The Sarin attack on the Tokyo subway, usually referred to in the Japanese media as the Subway Sarin Incident (地下鉄サリン事件 Chikatetsu Sarin Jiken?), was an act of domestic terrorism perpetrated by members of Aum Shinrikyo on March 20, 1995.In the confined space of the Tokyo subway, those attacks generated an average body count of 2.5 people per attack. In the open air environment of a marathon, I'd be surprised if the casualty count were higher. During WWI, troop casualties from chemical weapons rivaled those from high explosives because soldiers were entrenched, and heavier than air gases like most chemical weapons tended to find the lowest point in the ground, where it pooled along with the soldiers sheltering there from bullets and artillery shrapnel. Any attempt to exit the trenches was met by machine gun fire from the enemy.In five coordinated attacks, the perpetrators released sarin on several lines of the Tokyo Metro, killing thirteen people, severely injuring fifty and causing temporary vision problems for nearly a thousand others. The attack was directed against trains passing through Kasumigaseki and Nagatachō, home to the Japanese government. It is the most serious attack to occur in Japan since the end of World War II.