Although reports are unconfirmed that authorities are investigating the possibility that a welder’s torch may have created the spark that caused the blast, a spokesman for the project owner, Kleen Energy Systems, said that police were still treating the site as a crime scene.

The accident occurred just three days after the U.S. Chemical Safety Board approved safety recommendations on gas purging after a months-long investigation into a similar explosion at a ConAgra Slim Jim production facility in North Carolina.

“We haven’t been allowed on site yet to know one way or another [what caused the explosion],” says Daniel Horowitz, spokesman for the Chemical Safety Board. “But clearly gas purging is a very serious issue for us.”

The blast at the Kleen Energy Systems plant occurred shortly after 11 a.m. Sunday morning, causing severe structural damage to the 620 MW combined cycle electric generating facility that was being built on a former feldspar quarry site near Hartford. The project, which began in 2007, was about 93% complete and has been undergoing testing since the summer. About 114 workers were on the site at the time of the explosion with many still inside the building. So far, five have been confirmed dead and six remain hospitalized, with one in critical condition, according to a spokesman for O&G Industries, Torrington, the general contractor on the project. Search and rescue operations were ongoing as of Monday.

According to a January 15 report filed by Kleen Energy Systems with the Connecticut Department of Public Utility Control, construction activity over the last quarter has centered around the installation, hanging and welding of pipe throughout the site. The amount of welding required had exhausted the supply of available union members in Connecticut, requiring O&G to bring in workers from New York and Massachusetts. At the time the report was filed, 760 workers were on site and the project was considered to be ahead of schedule.

“We have all of our technical people looking at this and trying to analyze what the implications are both short and long term,” says DPUC spokesman Phil Dukes.

Middletown Mayor Sebastian Giuliano said Sunday the 800- to 1,000-ft natural gas line that runs through the building was being purged at the time of the explosion, but the CSB is cautioning against linking the explosion with purging a gas line until their investigation is complete.

“The CSB’s investigative team will examine the activities that were ongoing at the time of this accident, including any gas purging, as indicated by initial reports,” CSB lead investigator, Don Holmstrom, said in a statement.

Last week the CSB approved safety recommendations in response to the June 9 natural gas explosion at the North Carolina ConAgra facility that left four dead, three critically injured and sent a total of 67 to the hospital. The recommendations urged the National Fire Protection Association and the American Gas Association to enact changes to the National Fuel Gas Code requiring purged fuel gasses to be directly vented to an outdoor location, away from workers and ignition sources. The CSB’s investigation into the ConAgra incident determined that the explosion was caused by the accumulation of significant amounts of natural gas that had been purged indoors from a new 120-ft pipe during the startup of a new water heater.

In a statement released last week, Holmstrom referred to the purging of flammable gasses into building interiors as a “recipe for disaster.”

Energy Investors Funds, which acquired an 80% indirect investment in the plant in 2008 declined to comment specifically on the explosion, saying only that they are cooperating with authorities and their thoughts are with the workers and their families.

“We strongly value their contributions, efforts and dedication,” the company said in a statement.”