A dozen or more others were hurt, which was so powerful it alarmed residents who heard the boom and felt tremors in their homes miles away from the Kleen Energy Systems plant in Middletown, about 20 miles south of Hartford.
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 at the site, which is near Wesleyan University on a wooded and hilly 137-acre parcel of land overlooking the Connecticut River.
Search and rescue crews were combing through the debris from the damaged plant overnight but believed no one was missing amid the rubble, Deputy Fire Marshal Al Santostefano said. The investigation into what caused the explosion was to begin Monday morning, he said.
The U.S. Chemical Safety Board, a federal agency that investigates industrial chemical accidents, was mobilizing a team of workers from Colorado and hoped to have them on the scene by midday Monday, spokesman Daniel Horowitz said.
The nearly completed 620-megawatt plant is being built to produce energy primarily using natural gas, which accounts for about a fifth of the nation's electricity. Workers for the construction company, O&G Industries, were purging a gas line, clearing it of air, when the explosion occurred around 11:15 a.m. Sunday, Santostefano said.
About 50 to 60 people were in the area at the time, he said.
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."
Lynn Hawley, of Hartland, Conn., said her 36-year-old son, Brian Hawley, is a pipefitter at the plant and broke his leg. She said he called her from his cell phone to say he was being rushed to a 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."
Hospital officials didn't immediately release the conditions of the other injured people, whose wounds ranged from minor to very serious.
The thundering blast shook houses for miles.
"I felt the house shake," Middletown resident Steve Clark said. "I thought a tree fell on the house."
Mayor Sebastian Giuliano said he heard it as he was leaving church.
"It felt almost like a sonic boom," he said.
Kleen Energy Systems LLC began construction on the plant in February 2008. It had signed a deal with Connecticut Light and Power for the electricity produced by the plant, which was scheduled to be completed by mid-2010 and would be one of the biggest built in New England in the last few years.
The company is run by former City Councilman William Corvo. A message left at Corvo's home was not returned Sunday. Calls to Gordon Holk, general manager of Power Plant Management Services, which has a contract to manage the plant, also weren't returned.
Energy Investors Funds, a private equity fund that indirectly owns a majority share in the power plant, said it was cooperating with authorities investigating the explosion. In a written statement, the company offered sympathy and concern and said it would release more information on the explosion as it becomes available.
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 recently 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.
Gov. M. Jodi Rell visited the scene Sunday and announced late in the day that the state had imposed a temporary no-fly zone for a three-mile radius around the site to ensure that the safety of the search and rescue workers would not be jeopardized. The restrictions were put in place until Monday evening.
The state's Emergency Operations Center in Hartford also was activated, and the Department of Public Health was called to provide tents for shelter and medical triage.