The 400,000 cubic feet of natural gas blown through the pipes was released into the air in tight quarters, creating an explosive mixture large enough to fill a professional basketball arena, said Donald Holmstrom, lead investigator for the U.S. Chemical Safety Board.
With the U.S. building more natural gas power plants, he said, it is important to develop standards and new methods - such as cleaning pipes with air, steam or other substances - to prevent similar accidents.
"Thousands and thousands of workers across the country will be involved in constructing these plants. The safety of these workers and the nation's energy independence are at stake as these gas-fired plants are built over the next 20 years," he said.
The Middletown explosion comes as power generators are increasingly relying on natural gas to produce electricity because it's plentiful and cleaner than coal, and the use of gas is growing to power factories and heat homes.
Gas is used to make about a fifth of America's electricity. That's expected to grow to 26 percent by 2018 as older, coal-fired plants are retired. There are 35 gas-fired power plants in the U.S. either under construction or built but not yet operational.
The explosion ripped apart the nearly completed 620-megawatt Kleen Energy Systems plant as workers for O&G Industries Inc. purged the gas line.
Exactly what sparked the blast has not been determined, though Holmstrom said there were "several" potential ignition sources nearby.
He would not say what they were, but said that in general, natural gas can be sparked by anything from welders' tools and electrical devices to static electricity.
Several other industrial accidents involving natural gas have occurred in recent years, including one that killed four people last year the ConAgra Foods Inc. plant in Garner, N.C.; and another in 1999 at a Ford Motor Co. power plant in Dearborn, Mich., that killed six.
"Companies must ensure that flammable gases are not vented into close proximity with ignition sources and workers. That is a vital safety message from all these tragedies," Holmstrom said.
The Middletown blast, heard and felt for miles, occurred about an hour after some workers at the site complained of a heavy gas smell.
While some victims' relatives and blast survivors questioned safety standards at the plant and workers' long hours, other workers said the job was handled safely. The plant was scheduled to begin operation in June.
The U.S. Chemical Safety Board investigates serious chemical accidents. State and local authorities have interviewed survivors and sifted through debris for evidence as they conduct a separate investigation into whether there was any criminal negligence.
Holmstrom said the board is examining how the accident could have been prevented; determining exactly what ignited the gas at the Middletown plant is not a major focus of the its investigation.
"Our focus is not on finding out blame," Holmstrom said.
Middletown South District Deputy Fire Chief Marc Fongemie said Thursday that its investigation is continuing. State and local police and the federal Occupational Safety and Health Administration also are investigating.