Workers at a natural gas plant under construction were purging gas lines when a powerful explosion blew off part of the building Sunday, killing multiple people and injuring at least 14, officials said.

Betsy Hard, deputy commissioner of the state Department of Emergency Management and Homeland Security, told The Associated Press that multiple people died but she did not know how many.

Al Santostefano, the deputy fire marshal in Middletown, said 50 construction workers were in the section of the power plant where the explosion happened at 11:17 a.m. He said he did not know what caused the explosion.

Dogs were searching for victims in the rubble, but there were no signs of life Sunday afternoon, he said.

"They are taking the building apart piece-by-piece now, the part that collapsed and came in, they are taking that apart in sections piece-by-piece, very carefully," he said.

The 620-megawatt plant was being built to produce energy primarily using natural gas. Santostefano said workers for the construction company, O&G Industries, were purging the gas lines, a procedure he called a "blow-down," when the explosion occurred.

Middlesex Hospital transferred one seriously injured person to Hartford Hospital and was evaluating another Sunday afternoon who might also be moved to Hartford for more intensive care, said Middlesex Hospital spokesman Brian Albert.

Two people were treated and released, and eight others were being treated for broken bones, abdominal injuries, blunt force trauma and other kinds of injuries consistent with being caught in an explosion, Albert said.

They did not expect to receive any more patients, he said Sunday afternoon, about four hours after the explosion.

A nursing supervisor from St. Francis Hospital and Medical Center in Hartford said they had no patients from the explosion, and Hartford Hospital had two that were brought directly there after the blast, in addition to the one transferred later from Middlesex.

Several ambulances were seen leaving the scene of the blast without their lights on.

The powerful blast shook houses for miles.

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

Barrett Robbins-Pianka, who lives about a mile away and has monitored the project for years, said she was running outside and heard what she called "a tremendous boom."

"I thought it might be some test or something, but it was really loud, a definite explosion," she said.

Kleen Energy Systems LLC began construction on the power plant in February 2008.

The company is run by president and former Middletown City Council member William Corvo. A message left at Corvo's home was not immediately returned.

Calls to Gordon Holk, general manager of Power Plant Management Services, which has a contract to manage the plant, weren't immediately returned.

Gov. M. Jodi Rell was on her way Sunday afternoon to the site after speaking with Middletown Mayor Sebastian Giuliano, and called out a specialized search and rescue team to help firefighters.

The state's Emergency Operations Center in Hartford also was activated, and the Department of Public Health was called to provide tents at the scene for shelter and medical triage.