American authorities have launched a criminal investigation into what caused a deadly explosion at an unfinished Connecticut power plant that killed five construction workers, including one Canadian, according to reports.

More than a dozen others were injured in the blast, which occurred on Sunday morning.

Workers were attempting to purge a gas line at the time of the blast. It blew apart large sections of the Kleen Energy Systems plant in Middletown, Conn., about 30 kilometres south of Hartford, the state capital.

"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."

Parts of the plant were so badly damaged that investigators were unable to work in them on Monday.

Everyone who was assigned to work at the plant when the explosion occurred has been accounted for, officials said, suggesting that the number of deaths and injuries may not rise.

The explosion was felt as far away as North Branford, some 28 kilometres away, according to a report in the Hartford Courant.

Earlier Monday, the state governor, M. Jodi Rell, said officials were still sorting out who was on site at the time of the blast.

"There are a number of contractors who do the work at the building," Rell told WTNH-TV on Monday morning.

The blast

The thundering explosion left metal walls peeling off the plant's sides, and much of the structure was left blackened and surrounded by debris.

Santostefano said there were "piles of rubble everywhere, debris everywhere. In some places the debris is as high as 10 feet," or three metres.

It shook houses throughout Middleton, a college town of 48,000 that is home to Wesleyan University.

Mayor Sebastian N. Giuliano heard the blast as he was leaving church.

"It felt almost like a sonic boom," he said.

The cause of the blast remains under investigation.

The U.S. Chemical Safety Board, a federal agency that investigates chemical accidents, has sent a team to the site from Colorado.

The 620-megawatt plant was being built to produce energy primarily using natural gas.

It had been under construction for two years and was scheduled to be completed within a few months.

With files from The Associated Press