Workers at a Pennsylvania chocolate factory smelled rotten eggs before a powerful natural gas explosion that leveled one building and heavily damaged another, killing seven people. That’s according to federal safety officials, who released a preliminary report on the March 24 blast. The National Transportation Safety Board’s five-paragraph account Tuesday of the fatal explosion confirmed earlier reporting by The Associated Press and other media outlets that employees had detected an odor of natural gas at the R.M. Palmer Co. factory in West Reading. Federal investigators have been focusing on the role of a natural gas pipeline as they look for the cause of the explosion.