MIDDLETOWN, Connecticut (AFP) - A team of investigators searched Monday for the cause of a massive explosion that killed at least five workers at an unfinished energy plant in the northeastern US. Safety concerns forced rescuers to abandon overnight efforts to sift through the rubble and track down an unknown number of people still unaccounted for after the blast at the plant near Middletown in the state of Connecticut. Deputy fire marshall Al Santostefano told AFP beams were being erected to support the damaged structures at the plant so the rescue operation could be relaunched with sniffer dogs on hand to help locate any trapped survivors. We are still not sure how many people were there yesterday. There are quite a few different contractors, he said, adding that the toll for now was unchanged at five dead. In Washington, Secretary of Labor Hilda Solis said inspectors from the Occupational Safety and Health Administration had arrived on site to conduct a comprehensive investigation. The safety and health of workers is of paramount importance to me and to the US Department of Labor, Solis assured. The US Chemical Safety Board (CSB) also said it would send a seven-person team to the blast site. The team was expected on the scene later Monday, the independent investigative agency said in a statement. The CSBs investigative team will examine the activities that were ongoing at the time of this accident, including any gas purging, as indicated by initial media reports, lead investigator Don Holmstrom added. Authorities have ruled out terrorism as the cause of the blast shortly before midday Sunday, which was so huge that is broke the windows of nearby residential buildings and shook houses kilometers away. It was said to have occurred during a test procedure on one of the gas pipes at the Kleen Energy plant, a giant billion-dollar complex that had been due to go online later this year. Rescue workers helped by search dogs scoured the rubble late Sunday at the plant where a brief, but fierce fire following the accident sent flames and black smoke billowing skyward. There was like a fireball going up and a lot of smoke. The explosion was strong enough to break one of our windows. Our neighbors had also their windows destroyed, said Scott Harmann, 44, whose father lives in a house just across the Connecticut River from the plant. Nearby resident Mike Woronoff said he heard a loud boom at his house some 3.2 kilometers from the 620-megawatt plant. I have friends that live 15 miles from here that called me because they could hear it. Then we could see the smoke. It went on for a mile and a half, then stopped, he said. Uncertainty as to the final toll seemed set to continue until contractors working on the site are able to compile an accurate roster of those present. A Middletown fire official said it was initially thought there was approximately 50 employees there at the time and that its unknown how many people are missing. Sebastian Giuliano, the mayor of Middletown, told a press conference on Sunday that there could be anywhere from 100 to 200 people working on the site on any given day. The American Red Cross set up a phone number for concerned relatives.