  BAGHDAD, Iraq - A series of at least 12 explosions echoed throughout central Baghdad on Saturday night. Their cause was not immediately apparent. The blasts appeared to be targeting the fortified Green Zone enclave that houses the U.S. Embassy and Iraqi interim government buildings. An air raid siren sounded three times in the enclave over the span of an hour. "The siren came on, they said take cover. I have no idea what it is," said Bob Callahan, chief spokesman for the U.S. Embassy. The U.S. military had no immediate comment. 
