Middletown Mayor Sebastian Giuliano said at a late-afternoon news conference that at least 12 people were injured.
Deputy Fire Marshal Al Santostefano told The Associated Press before a news conference that crews were still searching for survivors in the rubble at the Kleen Energy Systems plant in Middletown, about 30km south of Hartford.
Santostefano earlier said about 50 people were in the area around 11:17am, when the explosion occurred. The mayor said at the news conference it was difficult to tell how many people were at the plant because multiple contractors were working on it with their own employee lists.
"They're trying to figure out who was on the job today, and where are they now?" Giuliano said.
The 620-megawatt plant was 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 AP 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 late Sunday afternoon, but hospitals reported some seriously injured patients.
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.
Kleen Energy Systems LLC began construction on the power plant in February 2008. It had signed a capacity deal with Connecticut Light and Power for the electricity produced by the plant. Construction 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.
Plants powered by natural gas are taking on a much larger role in generating electricity for the US. 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.
