By JOHN CHRISTOFFERSEN, and

PAT EATON-ROBB,Associated Press Writers

MIDDLETOWN, Conn. — An explosion that sounded like a sonic boom blew out walls of an unfinished power plant and set off a fire during a test of natural gas lines Sunday, killing at least five workers, injuring a dozen or more and leaving crews picking through debris for more possible victims.



At least 12 people were injured in the explosion at the Kleen Energy Systems plant in Middletown, about 20 miles south of Hartford. Crews with dogs were still searching the rubble as darkness fell over the plant, on a wooded hill along the Connecticut River.



It wasn't clear how many people, if any, were still buried. Deputy Fire Marshal Al Santostefano told The Associated Press that 50 to 60 people were in the area at the time of the explosion, but authorities said multiple contractors were working on the project, making it difficult to pinpoint how many people were missing.



"I think a majority of them did survive," Santostefano said. "Most of them did walk away."



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. Rescue crews had set up several tents alongside the site.



The explosion happened around 11:15 a.m., Santostefano said. Mayor Sebastian Giuliano, who heard the blast, called it a gas explosion but said the exact cause wasn't immediately clear.



"It felt almost like a sonic boom," Giuliano said at an evening news conference.



The search was focusing in part on who was at the plant at the time of the explosion. Giuliano said 100 to 200 workers would have been there on a typical weekday.



"They're trying to figure out who was on the job today and where are they now," Giuliano said.



One of those killed was Raymond Dobratz, a 57-year-old plumber from Old Saybrook, said his son, Eric Dobratz, who called the elder man "a great dad."



The 620-megawatt plant, which was almost complete, is being built to produce energy primarily using natural gas. Santostefano said workers for the construction company, O&G; Industries, were purging the gas lines, a procedure he called a "blow-down," when the explosion occurred.



The building was still standing, but the blast blew out the sheet metal that covers its sides.



Lynn Hawley, 54, of Hartland, Conn., told The Associated Press that her son, Brian Hawley, 36, is a pipefitter at the plant. He called her from his cell phone to say he was being rushed to Middlesex 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."



She said he had a broken leg and was expected to survive.



Officials had not released the conditions of the other injured people by Sunday evening, although they said at least a dozen people had injuries ranging from minor to very serious.



The thundering blast shook houses for miles.



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



Barrett Robbins-Pianka, who lives about a mile away and has monitored the project for years, said she was running outside and heard what she called "a tremendous boom."



"I thought it might be some test or something, but it was really loud, a definite explosion," she said.



Work on the plant was 95 percent complete, the mayor said.



Kleen Energy Systems LLC began construction on it in February 2008. It had signed a capacity deal with Connecticut Light and Power for the electricity produced by the plant, which was scheduled to be completed by mid-2010.



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



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



Plants powered by natural gas are taking on a much larger role in generating electricity for the U.S. Gas emits about half the greenhouse gases of coal-fired plants and new technology has allowed natural gas companies to begin to unlock gas supplies that could total more than 100 years at current usage levels.



Natural gas is used to make about a fifth of the nation's electricity.



Gov. M. Jodi Rell visited the scene Sunday; she earlier called out a specialized search and rescue team to help firefighters.



The state's Emergency Operations Center in Hartford also was activated, and the Department of Public Health was called to provide tents at the scene for shelter and medical triage.



Rell said the emergency teams were expected to work through the night and into Monday.



Daniel Horowitz, a spokesman with the U.S. Chemical Safety Board, said the agency is mobilizing an investigation team from Colorado and hopes to have the workers on the scene Monday.



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.



The board voted last week to recommend that national and international code writers strengthen their guidelines to require outdoor venting of gas lines or an approved safety plan to do it indoors.



In February 2009, an explosion at a We Energies coal-fired power plant near Milwaukee burned six workers. The Occupational Safety and Health Administration is still investigating.



In the past few years, an explosion at a Dominion Virginia Power coal-fired plant in Massachusetts killed three workers in November 2007, while one worker and nine others were injured at an American Electric Power plant of the same type in Beverly, Ohio, in January 2007.



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