Middletown, Connecticut (CNN) -- Federal investigators attempted Monday to launch an investigation into an explosion that tore apart a power plant under construction here Sunday, killing five people and injuring 12, but were rebuffed by local authorities.
Middletown police and the fire marshal blocked the Chemical Safety Board team from entering the blast site, and lead investigator Don Holmstrom was barred from interviewing witnesses, board public affairs director Daniel Horowitz said.
"They were told it was a crime scene," said Horowitz, who said that was not a valid reason and the board should have been granted access.
In fact, acting police Chief Patrick McMahon said a search warrant was issued for the site because criminal acts of negligence had not been ruled out. As a result, the site is considered a crime scene.
Middletown Mayor Sebastian Giuliano was unfazed by the board's complaints about access.
"They are a civilian agency and are in charge of making policy -- as I know it -- and, until the site is cleared, they will be in the way," he said. "Once the site is clear, they can go in."
Horowitz said the federal investigators had complained to the governor's office.
"It's essential we access a site quickly," he said. "Evidence can be lost, witnesses' memories may fade. We may not get an accurate picture the longer they delay access."
A spokesman for Governor M. Jodi Rell would not comment on the record.
Rell has announced that she is assembling a panel of agencies to identify the cause of the explosion and forming a second group of state agencies, local officials and subject-matter experts to determine whether state law should be changed.
"Our response to the Middletown explosion must follow two distinct but critical paths," Rell said in a news release. "We must first identify what went wrong and then determine every measure we can take to prevent future catastrophes. The reviews must be thorough, impartial and swift. And if there are concrete steps we can take in the meantime, we must be equally swift in putting those new measures into place."
The disagreement came as Giuliano said workers may still be missing. "You're not sure until you've searched everything," he said, noting that some of the structure is unstable and too dangerous to search.
Giuliano said it was difficult to account for everyone because multiple contractors were involved in the project, which was about 95 percent complete at the time of the explosion.
"Obviously, something went wrong," Giuliano said of the largest construction project in New England.
"It's like Chernobyl," he said, referring to the 1986 nuclear power disaster in what was then the Soviet Union.
"It doesn't look like [there is] a part of it that isn't damaged," he said about the Middletown project.
Giuliano said Sunday that the explosion occurred as plant workers were purging a natural gas pipeline.
Kleen Energy Systems said on its Web site that the plant would be gas-fired but would be able to use other fuels. It was expected to produce 620 megawatts of power and would be one of New England's largest power plants.
The plant had been expected to go online this summer, Giuliano said.
The dead were identified by authorities as Peter Chetulis of Thomaston, Connecticut; Ronald J. Crabb of Colchester, Connecticut; Raymond Dobratz of Old Saybrook, Connecticut; Roy Rushton of Hamilton, Ontario, Canada; and Chris Walters of Florissant, Missouri.
Dobratz, a 58-year-old pipefitter, leaves a wife, three children and five grandchildren.
"He loved his job," said his sister, Marie Heck. Asked whether he had expressed concerns about safety at the Middletown site, she said, "That was never an issue."
Residents up to 20 miles away reported hearing the blast about 11:19 a.m. ET Sunday .
"It felt like the house was shaking," said Peter Moore of Durham, about 10 miles away. He initially thought that there had been a traffic accident on his street or that there was a problem with his house, he said.
Moore said his mother, who lives in Woodbridge, about 20 miles from the plant, told him it "sounded like someone pounded on the back door a couple of times."
Some workers reported having been thrown as far as 40 feet by the blast.
On Thursday, the Chemical Safety Board had issued "urgent recommendations" that national fuel gas codes be changed to improve safety when gas pipes are being cleared of air during maintenance or the installation of new piping.
The recommendations resulted from the board's investigation into June's natural gas explosion at the ConAgra Slim Jim production facility in Garner, North Carolina, which caused four deaths and sent 67 people to hospitals.
CNN's Susan Candiotti contributed to this story.