It is unclear if this is an accident or a deliberate attack, but I would lean towards deliberate. Once the dust settles we'll see exactly what this was and perhaps what it means. There are a number of potential industrial chemicals such as chlorine that terrorists can use a weapon. Chlorine is one chemical that has been used ineffectively several times by several groups. These guys got it right, or they just got lucky.

http://www.cnn.com/2007/WORLD/meast/...ain/index.html

BAGHDAD, Iraq (CNN) -- A cloud of deadly toxic gas engulfed an Iraqi town Tuesday, killing six people and leaving dozens of others choking on fumes after a tanker carrying chlorine exploded outside a restaurant.

An Iraqi Interior Ministry official said the blast in the town of Taji, 12 miles (20 km) north of Baghdad, was caused by a bomb on board the tanker.

There were contrasting figures on the casualty toll. Baghdad security plan spokesman Gen. Qassim Atta told state-run al-Iraqiya TV that five people died in the blast and 148 were poisoned by the gas.