By Douglas Healey, Getty Images An explosion damaged the Kleen Energy Systems plant in Middletown, Conn. The 620-megawatt plant, under construction since 2008, is being built to produce energy primarily using natural gas.

A section of an under-construction power plant that exploded was too unstable to search Monday, and left questions about whether anyone was trapped inside because authorities do not have an exact roster of everyone who was on duty at the time of the blast.Sunday morning’s blast at the Kleen Energy Systems plant in Middletown, about 20 miles south of Hartford, killed at least five people and injured a dozen or more others. It happened as workers were clearing gas lines of air, but the exact cause remained under investigation. 

“I lost a couple of good friends up there,” Michael Rosario, a business representative with the local Plumbers and Pipefitters union, said as he broke down crying Monday. “I’m just so sorry something like this happened. They’re all great people. We’re all brothers and sisters.” 

He added, “There’s still a lot of unanswered questions.” 

Welders and other workers were at the site Monday, preparing to make it safer for emergency personnel, said Ed Reilly, president of the Greater Hartford-New Britain Building Trades Council. 

Middletown Deputy Fire Marshal Al Santostefano said officials have verified the whereabouts of 95% of the nearly 100 workers who were at the plant, citing conversations with contractors and labor union officials. It was unclear whether the workers who remain unaccounted for are missing or haven’t been contacted yet by authorities. 

Investigators returned to the scene Monday to try to begin determining the cause. 

Santostefano said he didn’t know when rescue crews would be able to search the small section of the plant that is unstable. 

The explosion that sounded like a sonic boom blew out walls of an unfinished power plant and set off a fire during a test of natural gas lines Sunday, killing at least five workers, injuring a dozen or more and leaving crews picking through debris for more possible victims. 

“It felt almost like a sonic boom,” Mayor Sebastian Giuliano said. 

The force of the blast at Kleen Energy Systems powerplant blew windows out of houses across the Connecticut River in Portland. 

The blast could be felt as far as Cheshire, 12 miles away, Common Councilwoman Deborah Kleckowski said. 

The plant was still under construction and not online. 

Kleckowski said the blast blew the plant’s top and sides off, exposing its metal frame. 

“The explosion sucked the windows out of the homes,” she said. It also knocked out windows on the Middletown side of the river and at Connecticut Valley Hospital, a psychiatric facility, she said. 

“I felt the house shake, I thought a tree fell on the house,” said Middletown resident Steve Clark. 

Giuliano said there was no danger to the public from the explosion or from gases or smoke emanating from the scene. 

The plant has been under construction since 2008. The 620-megawatt plant is being built to produce energy primarily using natural gas. 

Public safety officials were unsure exactly how many workers were at the scene when the explosion occurred, the Middletown Fire Department said. 

Deputy Fire Marshal Al Santostefano said 50 to 60 people were in the area at the time of the explosion. Multiple contractors were working on the project, making it difficult to know how many people were missing. Giuliano said 100 to 200 workers would have been there on a typical weekday. 

Santostefano said workers for construction company O&G Industries were purging the gas lines when the blast occurred around 11:17 a.m. 

Kleckowski said she felt a “sinking feeling of tragedy” when she heard what happened. 

Among the steady stream of people who gathered across the river to see, people were walking around shaking their heads. They seemed “awestruck by the magnitude of the devastation,” Kleckowski said. 

“People are afraid, what if (the plant) was on line? There would have been more people there,” she said. 

Brian Albert, spokesman for Middlesex Hospital in Middletown, said 11 people were brought in. Two were treated and released, eight are being treated for broken bones and other injuries. One person was transferred to Hartford Hospital, he said. 

Hartford Hospital said two other people were brought there directly after the blast. 

Contributing: The Associated Press