Fire marshals yesterday were preparing to start their investigation into a massive explosion that rocked an under-construction power plant where gas lines were being tested, killing at least five people.

A dozen or more others were hurt in Sunday's blast, which was so powerful it alarmed residents who heard the boom and felt tremors in their homes miles away from the Kleen Energy Systems plant in Middletown, about 20 miles south of Hartford.

The explosion left huge pieces of metal that once encased the plant peeling off its sides. A large swath of the structure was blackened and surrounded by debris, but the building, its roof and its two smokestacks were still standing at the site, which is near Wesleyan University on a wooded and hilly 137-acre parcel of land overlooking the Connecticut River.