"We know that 12 individuals have been injured. Five individuals are known to have lost their lives," Middletown Mayor Sebastian Giuliano told a news conference, held at a school around 6 p.m. Sunday.

    However, Giuliano said the exact number of casualties was not yet available as it was not known how many workers were actually at the site when the explosion occurred in the main building of the Kleen Energy Plant at about 11: 30 a.m. local time.

    The natural gas explosion was assumed to be the main cause of the incident that blew apart the power plant, and the possibility of a terrorist attack has been ruled out, the mayor said.

    A further investigation is underway.

    The power plant is not fully operational yet and the explosion occurred during a testing procedure, according to the city official.

    Nearly 95 percent of the construction has been completed, and the plant is expected to start operation this summer, he added.

    No representative of Kleen Energy showed up at the press briefing.

    Earlier, Al Santostefano, the deputy fire marshal in Middletown, told Xinhua that some 50 construction workers were believed to be working on the site when the explosion occurred.

    "Multiple contractors were on site - adding to the confusion on the number of employees actually working on site," Mayor Giuliano told the press briefing.

    Usually, some 100 to 200 people from various contracting firms would be present on the site. However, it was Sunday and people working there could be fewer, Santostefano said earlier.

    The mayor conceded that local authorities had so far failed to establish contact with anyone that might have escaped from the building and survived.

    However, he insisted that it was impossible to determine how many people were unaccounted for or missing before workers' rosters from every contractor were thoroughly checked out and the search and rescue work on the blast site concluded.

    According to the Middletown Fire Department, a search and rescue team was currently on site with sniffer dogs in an attempt to locate people who might have been trapped under the debris.

    Department officials vowed to continue the search throughout the night and well into Monday, despite a chilly and windy weather that may bring outdoor temperatures down to minus 7 degrees Celsius in the night.

    The Sunday blast broke the usual tranquility at Middletown, as the shockwave was felt by people living some 32 km away, and billowing black smoke rising from the fire could be seen miles away.

    "I heard a thundering noise, and it felt like a small earthquake," a local resident working in a building about 1.6 km away from the plant told Xinhua.

    "I saw police cars and helicopters and fire engines coming in, and heard that ambulances from nearby eight towns were called," he added.

    Mayor Giuliano told reporters that some local houses reported damage similar to that inflicted by an earthquake.

    Police cordoned off the road about 800 meters away from the blast site, which was behind a small hill and virtually out of sight to media people flocking into town.

    Connecticut Governor Jodi Rell visited Middletown Sunday afternoon, government sources said. The governor had earlier activated the state's emergency operations center in Hartford, about 30 km north of Middletown, her office said.

    Connecticut lieutenant governor and several state senators and representatives were present at Sunday's press briefing.