June 4 — By Gilles Castonguay BRUSSELS (Reuters) - Belgian investigators found a nerve gas ingredient in letters addressed to the Belgian prime minister's office, and the U.S., British and Saudi Arabian embassies, officials said on Wednesday. Two postal workers were briefly hospitalized after being exposed to the chemicals. The brownish-yellow powder contained phenarsazine chloride, an arsenic derivative used in nerve gas, as well as hydrazine, an agent used as a rocket propellant, said Health Ministry spokeswoman Anne-Francoise Gally said. In the amounts contained in the letters, the two chemicals are not life threatening, but can cause irritation to the...