BOSTON -- Search and rescue crews were digging through debris at a Connecticut power plant for signs of life Sunday after a powerful gas explosion injured several workers and killed at least five people.

The explosion occurred at 11:26 a.m. on Sunday at a plant owned by Kleen Energy Systems on River Road in Middletown, according to fire commissioner David Gallitto.

A news release from the office of Middletown Mayor Sebastian Giuliano says at least five people are confirmed dead and 12 people are known injured from Sunday's explosion.

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

Gerald E. Dailey, a town council member, said that 51 workers were at the gas-fired plant Sunday when the explosion occurred. Dailey said the plant has been under construction for about two years and officials were planning to test the power-generating system Sunday.

A fire official in Middletown, Conn., said in the evening that no one is known to be missing after the blast. Deputy Fire Marshal Al Santostefano said that crews will spend all night Sunday going through debris but there is nobody firefighters believe is unaccounted for.

He says the investigation of the cause of the blast will not begin until Monday morning.

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 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.

"It's just horrible, horrible," said one man choking back tears. "All I know is I lost some union brothers, some close, close personal friends. It's horrible."

Brian Albert, a spokesman for Middlesex Hospital, said that 11 workers were transported to the hospital for treatment. One of the workers was flown by medical helicopter to Hartford Hospital, Albert said, and eight others were still being treated for various injuries, including broken bones and abdominal injuries. Two of the injured workers were discharged Sunday. Middlesex Hospital is the closest medical facility to the power plant.

An official at Hartford Hospital said that two people were being treated there as a result of the blast, although she could not specify if they were workers at the plant. Their conditions are unknown.

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.

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.