Officials unsure what caused blast at construction site By Tony Plohetski AMERICAN-STATESMAN STAFF Monday, November 3, 2003 An explosion with an unknown cause rocked a City of Austin construction site Monday afternoon, injuring four workers, one critically, and rattling the ground and buildings up to 50 yards away. Construction on the site, once home to a chemical company, had been halted this year after crews discovered a white substance that they said appeared suspicious. However, that chemical was determined not to be hazardous, and work resumed a couple of days later, said Fred Evins, division manager for the city's project...