MIDDLETOWN - (AP) - An explosion blew apart a power plant under construction as workers purged natural gas lines Sunday, killing at least five people and injuring a dozen or more in ablast that shook homes for miles.

At least 12 people were injured in the explosion at the KleenEnergy Systems plant in Middletown, about 20 miles south ofHartford.

Deputy Fire Marshal Al Santostefano tells The Associated Pressthat crews will spend all night Sunday going through debris butthere is nobody firefighters believe is unaccounted for.

The explosion happened around 11:17 a.m., he said. MayorSebastian Giuliano called it a gas explosion but said the exactcause wasn't immediately clear.

"It felt almost like a sonic boom," Giuliano said at anevening news conference.

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

The building was still standing, but the blast blew out thesheet metal that covers its sides.

Lynn Hawley, 54, of Hartland, Conn., told The Associated Pressthat her son, Brian Hawley, 36, is a pipefitter at the plant. Hecalled her from his cell phone to say he was being rushed toMiddlesex Hospital.

"He really couldn't say what happened to him," she said. "Hewas in a lot of pain, and they got him into surgery as quickly aspossible."

She said he had a broken leg and was expected to survive.

Officials had not released the conditions of the other injuredpeople by Sunday evening, although they said at least a dozenpeople had injuries ranging from minor to very serious.

The thundering blast shook houses for miles.

Gov. M. Jodi Rell visited the scene Sunday; she earlier calledout a specialized search and rescue team to help firefighters.

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

For Chopper 12 footage of the explosion, click NEXT under the video player or go to iO Extra on Channel 612.