MIDDLETOWN - - UPDATE (7:18 a.m.): Crews are returning to the Kleen Energy plant this morning to determine when rescuers can resume their search through the rubble for workers who remain unaccounted for.

Aerial Photo Of The Middletown Power Plant Explosion (BETTINA HANSEN / HARTFORD COURANT) The search was suspended at about 2:30 a.m. because the debris is unstable, said Middletown Deputy Fire Marshal Al Santostefano.

Lights were brought in and dogs were assisting rescuers, he said. But all were called out when it was determined that the rubble may be dangerous.

He said experts will determine when, and under what circumstances, the search can resume.

Gov. M. Jodi Rell said this morning that most of the approximately 100 people who were believed to be working at the plant Sunday have been accounted for.

In Middletown, however, Santostefano said that workers arriving at the plant because they are concerned about co-workers are actually helping investigators. Although they aren't being admitted to the plant, they are enabling investigators to eliminate them as possible victims, Santostefano said.

Five people died in Sunday's explosion, which occurred as workers were purging gas lines.

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(Earlier Report) - A devastating explosion that was heard and felt for miles destroyed a power plant Sunday morning as workers purged a natural gas piping system, killing at least five and injuring many more, emergency response officers said.

Homeowners miles away said the explosion, reported shortly after 11 a.m. at the Kleen Energy Systems power plant under construction on River Road, created a shock wave so intense that some mistakenly thought the central part of the state had experienced an earthquake.

A team of investigators from the federal government's Chemical Safety Board, charged with investigating industrial chemical accidents, was scheduled to arrive at the plant today. A board spokesman said the team will look for the cause of the explosion, with particular emphasis on the procedure used at the Kleen Energy plant to purge, or clear, the gas piping system.

The identities of those killed were not being released Sunday night, but family members confirmed the death of Raymond E. Dobratz, 58, a pipefitter from Old Saybrook.

"I lost my father today," said his son, David Dobratz, 32.

David Dobratz, also a pipefitter, had worked on the plant but said he hadn't been there recently. Raymond Dobratz had three children and five grandchildren and was a member of Connecticut Plumbers & Pipefitters Local 777, his son said.

Rescue officers worked throughout the day to prepare a casualty list, but the task was complicated by uncertainty over the number of workers on the site Sunday.

At 5:30 p.m., Mayor Sebastian Giuliano said there were five confirmed dead and as many as a dozen more injured, many suffering broken bones after being flung through the air.

Hours after the shocking blast, which blew out windows and cracked foundations of neighboring houses, state police with specially trained dogs continued to poke through the rubble of twisted steel.

"There are bodies everywhere," a witness said in the hours immediately after the explosion. Later in the afternoon rescue personnel said victims may still be buried in rubble.

Middletown Deputy Fire Marshal Al Santostefano said there were "confirmed fatalities" but that he did not know how many. Estimates placed anywhere from 50 to more than 100 workers on the site Sunday. Without a definitive list of who was working, emergency personnel had difficulty determining how many might be missing.

Authorities planned to search through the night for victims who might be trapped in debris.

"It was a massive explosion," Santostefano said

Eddie Reilly, president of the building trades council in Hartford, confirmed that there were about 50 tradesmen on site Sunday morning.

The plant was nearing completion. It was designed to generate electricity by burning natural gas.

There was a brief, intense fire immediately following the explosion. It was quickly extinguished by about 100 responding firefighters, witnesses said. There was no danger to the public by mid-afternoon, authorities said.

Santostefano said the explosion was related in some fashion to natural gas, but that the cause was still under investigation. He said the explosion appears to have occurred when operators attempted a "blow down" of natural gas pipelines, a procedure that involves the purging of gas pipelines.

Only three days ago, the federal Chemical Safety Board was considering what it called urgent recommendations to change national fuel gas codes to improve safety when gas pipes are being purged, or cleared of air during maintenance or installation of new piping.

The recommendations grew from the board's ongoing federal investigation into the June 9, 2009, natural gas explosion at the ConAgra Slim Jim production plant in Garner, N.C., which caused four deaths, three critical life-threatening burn injuries and other injuries that sent 67 people to the hospital.

Board investigators determined that the North Carolina explosion resulted from the accumulation of natural gas that had been purged indoors from a new 120-foot length of pipe during the startup of a new water heater in the plant that made beef-jerky products. During pipe-purging, workers feed pressurized gas into a pipe to displace air or other gases so that only pure fuel gas remains in the piping when it is connected to an appliance such as a water heater or boiler.

The Middletown explosion is among the most serious in a year, board spokesman Daniel Horowitz said Sunday night. He said investigators will look closely at whether the purging process contributed to the explosion.

"This is an issue the board is very concerned about," Horowitz said. "We don't know if there's any connection at this point."

The Middletown power plant site, carved into a rocky bluff over a bend in the lower Connecticut River, consisted of numerous structures. But Santostefano said he believes the explosion occurred in the largest, a massive, square steel structure known as the power block building.

Middletown Councilman Ronald P. Klattenberg said the explosion blew out all sides of the power block building.

"Parts of the walls are just flapping in the wind," Klattenberg said.

Santostefano said "They are taking the building apart, piece by piece. If they do find anybody, they would be under the rubble." He said rescue workers were in "search and rescue mode."

He said authorities believe many of those on the site at the time of the explosion worked for O&G Industries of Torrington, the general contractor building the plant, which was more than 95 percent complete.

Emergency response personnel poured into the site after the explosion, which was reported to the Middletown Fire Department at 11:19 a.m. Helicopters airlifted victims to area hospitals. Most were taken to Middlesex Hospital in Middletown.

Middlesex Hospital staff said they received 11 victims. They said one had head injuries and was transferred to Hartford Hospital; two were treated for minor injuries in Middlesex Hospital's emergency room and released.

Eight are still being treated for "multiple injuries" in the emergency room, said hospital spokesman Brian Albert.

The injuries include broken bones, orthopedic injuries and bruises. One victim sustained a fractured pelvis, another has a broken leg and several have internal injuries, hospital officials said.

"We expect that we will be admitting two or three of those patients," Albert said. Middlesex Hospital physician Jonathan Bankoff said the injuries were consistent with a blast; no burn victims were being treated at Middlesex.

Some of the victims were thrown 30 to 40 feet and suffered abdominal injuries and broken bones, Bankoff said at an afternoon press conference.

"The majority of our patients are telling that story," he said.

Public records associated with the Kleen Energy plant permitting process show that it was designed to generate electricity principally by burning natural gas to power a combined cycle turbine. Such turbines reuse waste heat produced during the power generation process, increasing the plant's efficiency.

The plant operators proposed that when sufficient supplies of natural gas were not available, the plant would operate on low sulfur fuel oil.

The project was proposed mostly for the benefit of power consumers in the Middletown area, according to records.

Kleen Energy Systems received approval to generate 520 megawatts - enough electricity for 364,000 to 520,000 households - in November 2002 from the Connecticut Siting Council. As of 2006, the company was petitioning the council to produce 620 megawatts. A megawatt is enough to serve 700 to 1,000 homes.

A resident of East Hampton across the river from the plant said he heard a loud booming explosion about 11 a.m. Immediately afterward, his house was hit with a concussion that caused him to believe someone had driven an automobile into his home.

The concussion interrupted services at a nearby East Hampton church, causing parishioners to speculate that the area had just experienced an earthquake.

Homeowners across the river from the plant in Portland said the concussion blew out windows and doors and cracked concrete structures.

Other witnesses said they felt the explosion as far away as North Branford and Durham.

A homeowner in Branford said, "My entire house shook, followed by what sounded like an explosion."

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

Courant Staff Writers Josh Kovner, Monica Polanco, Jenna Carlesso and Alaine Griffin contributed to this story.