The Hartford Courant reported a natural gas line had just been purged Sunday morning, an operation that can leave gas in the air until it is vented. A welding spark or industrial space heaters being used to heat the plant could have ignited the gas, sources told the newspaper.
The Kleen Energy Systems plant in Middletown was under construction and was expected to begin operation later this year.
Fran Walters of Florissant, Mo., lost her husband in the explosion. She said a police officer told her: "The building was full of gas and before they could do anything, it was too late."
The explosion also injured 26 people.
The building was being treated as a crime scene. Union members were allowed in Monday to escort the bodies of the dead from the plant.
Rep. Joe Courtney, D-Conn., visited the plant Monday.
"They moved in while we were getting briefed. It was such a moving scene. It hurt literally just seeing it," Courtney said of watching the union procession.