NEWPORT, Wash. - Eleven people are recovering from an explosion that put an end to operations at the Zodiac Aerospace plant in Newport on Tuesday.

Although the cause was initially unknown, first responders learned that a pre-preg treater machine was believed to have caused the explosion, according to a Pend Oreille County Sheriff's Office press release.

The machine is used at the plant to make laminate material for aircraft interiors. The plant employed 85.

The blast occurred shortly before 9 p.m. and caused one floor to collapse on top of another. Windows shattered and doors were blown off their hinges. Residents on the scene said they heard the blast from as far as a mile away.

Of the 15 employees working at the time, 11 were treated for injuries. Becky Hogue, 45, of Newport; Marla Chute, 51, of Newport; and Rebekah Chaney, 21, of Oldtown, were treated and released at Newport Hospital.

Two employees were transported via Life Flight to Sacred Heart Medical Center in Spokane.

Linda Bergerson, 68, of Oldtown, was listed in serious condition and Christina Giannone, 33, of Newport, was listed in stable condition.

Newport Administrator and Building Official Ray King estimated the explosion caused $1 million in damages to the plant.