MIDDLETOWN, Conn. (AP) — 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.

“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 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.

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.

“We needed something to lift spirits around here, and that definitely did it,” he said.

The men who died were identified by police as Peter Chetulis, of Thomaston; Ronald J. Crabb, 42, of Colchester; Raymond Dobratz, 58, of Old Saybrook; Chris Walters, 42, of Florissant, Mo.; and Roy Rushton, of Hamilton, Ontario.

Walters was the safety supervisor for Keystone Construction, the power plant’s electrical contractor, whose job was to make sure everyone complied with federal work safety rules, his widow, Fran Walters, told The Associated Press. Crabb was a foreman, his brother Carl Crabb told the News-Tribune of LaSalle, Ill.

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 U.S. 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.

Spokesman Daniel Horowitz said the board’s investigators will need to determine which federal safety standards apply to the Kleen Energy plant. He said the fuel gas code has an exemption for power plants.

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.

A team from the board on Monday was turned away by local authorities, who said they would have access once criminal investigators had cleared them.

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.”

Parker said he could not recall any recorded building code violations involving the project.

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

OSHA records show there was a planned inspection on July 28, 2009, for work being performed by O&G; Industries. There was one violation relating to record keeping and reporting. OSHA spokesman John Chavez said records show O&G; settled the matter informally by paying a $1,000 fine.

“Relatively speaking, they do appear to have a pretty clean record,” Chavez said.

Also Monday, Gov. M. Jodi Rell ordered a review of state safety codes. She created two panels, one to identify the cause of the explosion and contributing factors, such as construction problems, worker safety issues and licensing or permitting matters. The other, a panel of state agencies, local officials and experts, will be charged with determining whether changes need to be made to Connecticut laws, state or local regulations or building or fire codes.

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

Middlesex Hospital spokeswoman Melissa Brady said all the injured were expected to survive.

Kleen Energy Systems LLC began construction on the plant in February 2008 on a wooded and hilly 137-acre 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 the nation’s electricity.

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Associated Press writers Susan Haigh and Pat Eaton-Robb in Hartford and Cheryl Wittenauer in St. Louis contributed to this report.