Smoke rises from the Kleen Energy plant in Middletown, Conn., on Sunday after an explosion that officials said killed at least five people and injured at least 12 others. (Mick Bolduc/The Courant/Associated Press)

Five people are known to be dead and at least 12 injured following a powerful explosion Sunday at a natural gas plant under construction in Middletown, Conn., the mayor's office said.

Middletown deputy fire marshal Al Santostefano said crews were still searching the rubble for survivors of the explosion, which happened at 11:17 a.m. ET.

"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," Santostefano said.

Mayor Sebastian Giuliano said it's difficult to tell how many people were in the power plant at the time of the blast because multiple contractors were working on it and had their own employee lists.

Al Santostefano, deputy fire marshal for Middletown, Conn., speaks to reporters on Sunday. (CBC)Officials had said earlier that 50 construction workers were in the section of the plant where the explosion happened.

Santostefano said he didn't know what caused the blast, which shook houses up to 16 kilometres away.

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

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

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.

Hartford Hospital said two patients were brought directly there after the blast, in addition to the one transferred later from Middlesex.

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

Connecticut Gov. M. Jodi Rell visited the scene after calling 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.

Rell said the emergency teams were expected to work through the night and into Monday.

The U.S. Chemical Safety Board is mobilizing an investigation team from Colorado and hopes to have the workers on the scene Monday, spokesman Daniel Horowitz said.

Safety board investigators have done extensive work on the issue of gas line purging since an explosion last year at a factory in North Carolina killed four people.

Just last week, the board voted to recommend that national and international code writers strengthen their guidelines to require outdoor venting of gas lines or an approved safety plan to do it indoors.

With files from The Associated Press