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 and injuring a dozen or more.

The explosion at the Kleen Energy Systems plant in Middletown, about 20 miles south of Hartford, could be heard and felt for miles.

Deputy Fire Marshal Al Santostefano told The Associated Press on Sunday night that no one was known to be missing amid the rubble of the damaged plant. 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 had set up several tents alongside the site, which is a few miles from Wesleyan University.

The explosion happened around 11:15 a.m., Santostefano said. Mayor Sebastian Giuliano heard the blast.

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

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. 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.

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.