An explosion 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. At least 12 others were injured in the explosion at the Kleen Energy Systems plant in Middletown, some 32 kilometers south of Hartford. Deputy Fire Marshal Al Santostefano told The Associated Press on Sunday night that no one was known to be missing. Still, crews planned to spend all night going through debris in case there were any more victims. The cause of the gas explosion was unknown, and the investigation was to begin Monday morning, he said. 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 set up several tents alongside the site. The explosion occurred Sunday morning, Mr. Santostefano said. Mayor Sebastian Giuliano heard the blast. It felt almost like a sonic boom, Mr. Giuliano said at an evening news conference. Mr. Santostefano said 50 to 60 people were in the area at the time of the explosion, and multiple contractors were working on the project, making it difficult to quickly account for everyone. 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. The 620-megawatt plant, which was almost complete, is being built to produce energy primarily using natural gas. Mr. 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 thundering blast shook houses for miles. I felt the house shake, I thought a tree fell on the house, said Middletown resident Steve Clark. 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. 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 coalfired 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 nations electricity. 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. 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.