A huge explosion rocked central Baghdad just be for sunset Thursday and a massive plume of black smoke was rising into the air along the east bank of the Tigris River. The blast occurred shortly after two heavy mortar shells slammed into the heavily fortified Green Zone. The public address system inside the zone, where the US Embassy is located, could be heard warning in English that people should take cover, "this is not a drill." Heavy gunfire rang out throughout the center of the city in the aftermath of the explosion, likely to have been a car rigged with explosives, thundered across the city and was especially heavy in the Karrada district. Sirens on emergency vehicles wailed through the region. Initial reports from police said two people were killed in the explosion.