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An underground steam pipe explosion ripped through a street in Midtown Manhattan during rush hour Wednesday, killing one person from an apparent heart attack and injuring about 30 others. The powerful blast spewed rubble into crowded streets, splashed mud on commuters, and sent a giant plume of murky hot steam billowing into the air. Fears of a terrorist attack rippled across the city as the explosion shook buildings and sent terrified people fleeing down stairways and into the streets. The blast, at 41st Street and Lexington Avenue near Grand Central Terminal, came from a broken 24-inch steam pipe that was installed in 1924.