MIDDLETOWN - Witnesses and emergency response authorities said as many as 100 people were injured and an undetermined number may have died when a massive explosion, which homeowners more than 10 miles away and mistook for an earth quake, blew up a power plant being built on the Connecticut River in the southern section of Middletown.

An hour after the explosion and what is believed to be the Kleen Energy Systems plant on River Road, emergency rescue personnel were continuing to arrive by vehicle and helicopter. Helicopters were airlifting victims to area hospitals.

Neighbors of the plant said as many as 100 employees may have been working there when the explosion took place. Confirmed information about damage and injuries from authorities was difficult to obtain.

A resident of East Hampton almost directly across the river from the plant said he heard a load booming explosion at about 11 a.m. Immediately afterward his house was hit with a concussion that caused him to believe someone had driven an automobile into his home. The concussion interrupted services at a nearby East Hampton church, causing parishioners to speculate that the area had just experienced an earthquake.

Other witnesses said they felt the concussion as far away as North Branford and Durham.

A neighbor of the plant said there were quantities of natural gas on the site.

A witness looking at the destruction from across the river in Portland said the main plant building seemed to have been substantially leveled.

State police fire and explosive investigators were rushing to the scene at around noon Sunday, as well as state police urban rescue crews, which would search rubble for victims.