MIDDLETOWN, Conn. (AP) — A power plant explosion that killed at least five people left a section of the building too unstable for rescue crews to determine whether everyone was accounted for, a fire official said Monday.

Piles of rubble were 10 feet tall in some parts of the plant, and mounds of rubble and debris were everywhere, said Middletown Deputy Fire Marshal Al Santostefano. On Sunday night, he had said no one was missing but by Monday 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 Monday morning that officials still haven’t received rosters of workers who were at the Kleen Energy Systems plant Sunday, and Santostefano said he didn’t know when emergency crews would be able to search the unstable area.

“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.”

Local fire investigators and federal authorities on Monday were expected to begin their investigation into what caused Sunday morning’s explosion at the plant in Middletown, about 20 miles south of Hartford.

A dozen or more others were hurt in the blast, which happened as gas lines were being tested. The explosion was so powerful it alarmed residents who heard the boom and felt tremors in their homes miles away.

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.

Rescue crews combed through the debris until about 2:30 a.m. Monday.

The U.S. Chemical Safety Board, a federal agency that investigates industrial chemical accidents, was mobilizing a team of workers from Colorado and hoped to have them on the scene by midday Monday, spokesman Daniel Horowitz said.

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. Workers for the construction company, O&G; Industries, were purging a gas line, clearing it of air, when the explosion occurred around 11:15 a.m. Sunday, Santostefano said.

About 50 to 60 people were in the area at the time, he said.

One of those killed was Raymond Dobratz, a 58-year-old plumber from Old Saybrook, said his son Erik Dobratz, who called the elder man “a great dad.”

Lynn Hawley, of Hartland, Conn., said her 36-year-old son, Brian Hawley, is a pipefitter at the plant and broke his leg. She said he called her from his cell phone to say he was being rushed to a hospital.

“He really couldn’t say what happened to him,” she said. “He was in a lot of pain, and they got him into surgery as quickly as possible.”

Hospital officials didn’t immediately release the conditions of the other injured people, whose wounds ranged from minor to very serious.

The thundering blast shook houses for miles.

“I felt the house shake,” Middletown resident Steve Clark said. “I thought a tree fell on the house.”

Mayor Sebastian Giuliano said he heard it as he was leaving church.

“It felt almost like a sonic boom,” he said.

Kleen Energy Systems LLC began construction on the plant in February 2008. It had signed a deal with Connecticut Light and Power for the electricity produced by the plant, which was scheduled to be completed by mid-2010 and would be one of the biggest built in New England in the last few years.

The company is run by former City Councilman William Corvo. A message left at Corvo’s home was not returned. Calls to Gordon Holk, general manager of Power Plant Management Services, which has a contract to manage the plant, also weren’t returned.

Energy Investors Funds, a private equity fund that indirectly owns a majority share in the power plant, said it was cooperating with authorities investigating the explosion. In a written statement, the company offered sympathy and concern and said it would release more information on the explosion as it becomes available.

Safety board investigators have done extensive work on the issue of gas line purging since an explosion last year at a Slim Jim factory in North Carolina killed four people. They’ve identified other explosions caused by workers who were unsafely venting gas lines inside buildings.

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Contributing to this report were Associated Press writers Eric Tucker in Middletown; Stephanie Reitz in Glastonbury, Conn.; Mark Williams in Columbus, Ohio; Mike Baker in Raleigh, N.C.; and Anne D’Innocenzio in New York.