Connecticut, United States – Authorities launched a criminal investigation Monday into the cause of an explosion that killed five people at a power plant under construction, saying they couldn’t rule out negligence.

The powerful explosion blew apart large swaths of the nearly completed 620-megawatt Kleen Energy plant as workers for the construction company, O&G Industries Inc., were purging a gas line Sunday morning. The blast tore apart sheet metal that covered the plant’s sides and left parts of the complex so unstable that rescuers were unable to work Monday because of the danger of collapse.

“If everything went right, we wouldn’t all be here right now,” Middletown Mayor Sebastian Giuliano said. “There’s a point where negligence raises to the level of criminal conduct, and that’s what we’re investigating.”

The mayor said rescue crews had been unable to get to all areas of the plant and he could not say for certain that no more victims would be found. But authorities also said everyone who was assigned to work at the plant at the time of the explosion was accounted for.

Deputy Fire Marshal Al Santostefano said the death toll should stand at five.

A representative of the local Plumbers and Pipefitters union, Michael Rosario, broke down crying as he talked about the men on Monday.

“We hug our families, kiss our children,” he said. “We go to work, and we want to come home at the end of the day, safe. That didn’t happen for a few people yesterday.”

It was still unknown what caused the blast. Middletown’s acting chief of police, Patrick McMahon, said police had ruled out any intentional act and were focusing on whether there was negligence.

Workers for O&G Industries, a Torrington-based general contractor, were clearing the gas lines of air when the explosion happened just outside the building, between two of the generators, Giuliano said.

During the procedure, local officials said, equipment such as welding machines and electricity should be shut off. Santostefano said officials don’t know if all equipment was shut down before the blast, and Giuliano said investigators will look into whether any equipment was on or anything that could have ignited the gas was at the scene.

Santostefano said workers were at the site Sunday, a few hours before the Super Bowl, because they were trying to get the plant, slated to open in the middle of 2010, open on time. He added: “It wasn’t like they were working in a frenzy.”

Officials from United Association Local 77, which represents plumbers and pipefitters who work at the site, did not comment Monday on what happened to cause the explosion. A spokeswoman for Energy Investors Fund, a private equity fund that indirectly owns a majority share of the power plant, would not comment.

The US Chemical Safety Board, a federal agency that investigates serious chemical accidents, last week issued urgent recommendations that national fuel gas codes be changed to improve safety when gas pipes are being purged.

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 town’s chief building inspector, John Parker, said there were “a lot of eyes” on the Kleen Energy project as it was being built. His office conducted numerous building inspections over the years — at times daily. He said third-party inspectors often were on hand.

Parker said it appears the workers were performing the purge by sending nitrogen through the line followed by natural gas to clear out any moisture, which he called “an accepted and approved method.”

Investigators from the Occupational Safety and Health Administration were also at the site.

O&G said about 114 workers for nine subcontractors were on the site at the time. It said six workers were still hospitalised Monday.

Kleen Energy Systems LLC began construction on the plant in February 2008 on a wooded and hilly parcel of land overlooking the Connecticut River, a few miles from Wesleyan University. It had signed a deal with Connecticut Light and Power for the electricity produced by the plant, which would be one of the biggest built in New England in the last few years.

The plant would produce energy primarily using natural gas, which accounts for about a fifth of America’s electricity.