Kevin White, a Middletown police officer, told Reuters news agency that he did not know how many people were on site at the time of the accident but there were "mass casualties."
Witnesses said the concussion of the blast felt "like an earthquake" and that black smoke bellowed out of the construction site.
Ambulances rushed to the site and helicopters were used to airlift some of the injured to hospitals.
Officials were scrambling to compile a list of workers on the site at the time of the blast in an effort to assess whether anyone else was still missing.
Deputy Fire Marshal Al Santostefano told the Associated Press about 50 to 60 people were in the area at the time of the explosion, and multiple contractors were working on the project, making it difficult to quickly account for everyone.
Meanwhile rescue workers helped by search dogs scoured the rubble left by the explosion.
The department of public health was providing tents for medical triage and shelter, with temperatures in the area near freezing point.