At least five people were killed and 12 injured on Sunday when a massive explosion rocked a large gas-fired power plant being built in Middletown, Connecticut, the town’s mayor said.

Fire officials said a natural gas leak caused the blast during testing at the Kleen Energy Systems LLC facility, which was 95 percent complete and due to come online this summer as the largest electricity generating plant in New England.

As many as 200 workers were at the site on any given day and the exact number of dead and injured would not be known until each contractor provided a list of employees, Middletown Mayor Sebastian Giuliano told a press conference.

Search and rescue teams with dogs and several fire departments were still sifting through the rubble after the explosion and fire, Giuliano said.

Kleen Energy Systems is majority-owned by Energy Investors Funds Group, a private equity firm.

Witnesses said flames shot up and black smoke billowed out when the blast shattered the Sunday morning calm. Some people reported windows blown out and the force was felt as far away as East Haven, a distance of 30 miles.

“The whole house shook. I didn’t know what it was, whether it was the house or the water heater or what,” said Cornelia Hull, who lives across the Connecticut river in Portland.

Dozens of ambulances from across the region and several helicopters rushed to the scene.

Eleven injured people were taken to Middlesex Hospital in nearby Middletown, said Peg Arico, manager of public relations at the hospital. Two were later discharged and one was sent to a hospital in the state capital, Hartford, which also received a patient directly from the scene.

Those admitted had injuries consistent with impact, including broken bones.

The explosion happened just before 11:30 a.m. during tests at the plant, a 620-megawatt gas-fired facility. The fire took about an hour to extinguish.

“It was connected with the gas, the natural gas that was coming into the building. That was part of the involvement,” said Al Santostefano, deputy fire marshal in Middletown.

Giuliano said there was no threat from substances getting into the atmosphere or of future explosions.

“SONIC BOOM”

The blast happened as workers were purging the gas lines, a process that is done repeatedly with pressurized air to remove debris.

Giuliano, who was coming out of church when he heard “a sonic boom,” said many of the workers were evacuated before the gas lines were purged but it was not yet clear how many remained in the building.

State emergency official Betsy Hard said local authorities had asked the state for help. The Department of Public Health put up tents at the scene for medical triage and shelter with temperatures below the freezing mark and brisk winds blowing.

The Kleen Energy plant was designed to operate on natural gas using a combined cycle turbine. Such turbines reuse waste heat produced during the generation process, increasing the plant’s efficiency.

Much of the plant’s power generation equipment was supplied by Germany’s Siemens AG and was being installed but was not yet operating, Siemens spokeswoman Melanie Forbrick said.

“This was a very tragic accident and our deepest sympathy goes out to all those who have been affected,” Forbrick said, adding the one Siemens employee working at the site was safe.

Contractor O&G Industries, of Torrington, Connecticut, is overseeing all local, state, and federal permitting, engineering and construction of the project.

There had been no safety violations or accidents previously reported at the site, Giuliano said.

He said it was not clear what had caused the gas to leak but he ruled out terrorism or vandalism.

“Nobody set a bomb off,” he said. “It wasn’t intentional.”

Reuters