MIDDLETOWN, Conn. (AP) - 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
from 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 on a wooded and hilly 137-acre parcel of land
overlooking the Connecticut River.

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.
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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 line when the explosion occurred.

Lynn Hawley, 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,"
Middletown resident Steve Clark said.

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 former Middletown 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 is fully
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.

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 at the scene for shelter and medical triage.

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.
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.

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.

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 November 2007, an explosion at a Dominion Virginia Power
coal-fired plant in Massachusetts killed three workers, and in
January 2007 one worker and nine others were injured at an American
Electric Power plant of the same type in Beverly, Ohio.