From the Debka File:
Western military sources have told DEBKA file that three chemicals were believed present in the Scud B rocket which exploded in the Aleppo neighborhood of Khan al-Assal Tuesday March 19: phosphorus, chlorine and Agent 15 or BZ. Although the Assad regime and the rebels charged each other with firing the rocket, which killed 15-31 people and injured more than a hundred, it was not possible to verify which side was actually responsible. The White House denied it was the rebels, while Moscow insisted that it was, in support of the accusation from Damascus.
The assumption in Israeli security circles is that either or both sides may have tried a one-shot use of a chemical weapon to test the limits of world-power tolerance. The incapacitating Agent 15 which causes choking is the least harmful of Assad’s chemical arsenal. A US army spokesman said the American armed forces had plans for intervening in the Syrian conflict if chemical weapons were used.