The explosion at the Kleen Energy Systems plant in Middletown, Connecticut, about 20 miles south of Hartford, killed at least five workers and injured a dozen or more on Sunday.
Crews spent the overnight hours going through debris in case there are any more victims.
The explosion blew out walls of the plant and set off a fire during a test of natural gas lines. The cause of the blast is unknown, and the investigation is to begin in the morning.
Mayor Sebastian Giuliano, who heard the blast while leaving church, says "It felt almost like a sonic boom."
Fire officials say multiple contractors were working on the project, making it difficult to quickly account for everyone.
The nearly complete 620-megawatt plant is being built to produce energy primarily using natural gas.