MIDDLETOWN, Conn. — A Connecticut fire official says about five people are unaccounted for after a deadly explosion at an under-construction power plant.

Sunday's explosion at the nearly completed plant in Middletown, about 20 miles south of Hartford, killed at least five people and injured a dozen or more. Officials say it happened as workers were clearing a gas line of air.

Middletown Deputy Fire Marshal Al Santostefano says it remains unclear whether the workers who are unaccounted for are missing or haven't been contacted yet by authorities.

Santostefano said today that 95 percent of the nearly 100 workers who were at the plant are accounted for, judging by conversations with contractors and labor union officials. But he says a section of the Kleen Energy Systems plant remains too unstable to see whether anyone is still trapped.

Authorities say different contractors were working at the site, making it difficult to determine how many people were there during the blast.

Piles of rubble were 10 feet tall in some parts of the plant, and mounds of rubble and debris were everywhere, Santostefano said. On Sunday night, he had said no one was missing but by this morning, he said, the extent of the damage was clearer and officials realized there was a section of building that could not be searched.

Gov. M. Jodi Rell said today officials still haven't received rosters of workers who were at the plant Sunday.

"There are a number of contractors who do the work at the building," Rell told WTNH-TV. "Until we actually have a roster of the names of those individuals that are in each of those groups and who was working on Sunday, we need that before we can do anything else. ... We're still confirming the number of people."

Santostefano added, "There's still uncertainly about who came in and who didn't come in yesterday."

The blast 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.

The nearly completed 620-megawatt plant is being built to produce energy primarily using natural gas, which accounts for about a fifth of the nation's electricity.