THOMASTON – The Cianbro employee injured in a 52-foot fall while working at Dragon Products Co. remained in stable but guarded condition Friday, according to a Cianbro spokeswoman.

Nicholas “Nick” Wark, 22, of Plymouth fell between a cluster of six silos, Rockland Fire Chief Charles Jordan Jr. said Friday.

In explaining the location of the accident, Jordan said the group of silos resembles the top of an open box of straws. In between are “interstitial spaces,” which is where Wark fell onto a surface with the consistency of gravel. The width of that space was estimated by rescuers at 15 to 20 feet.

It was not clear Friday what caused the accident.

Cianbro, based in Pittsfield, is one of the largest construction services companies on the East Coast and is involved in a $40 million expansion at Dragon, a sprawling operation alongside U.S. Route 1 near the Rockland line.

On Thursday, Rockland Fire Department’s confined-space entry team and Thomaston Fire and Emergency Medical Department personnel worked for two hours to remove Wark from the hole. They were assisted by Cianbro workers, who are also trained in confined-space rescues.

After Wark was taken to Central Maine Medical Center in Lewiston by a LifeFlight of Maine helicopter, he underwent surgery for internal injuries, Cianbro spokeswoman Dottie Hutchins said Friday.

It had been thought he would have to undergo a procedure to remove dust in his lungs that resulted from being in the confined space. Late Friday, however, Wark’s lungs began clearing without the procedure, she said, and his condition seemed to be improving.

Hutchins had not received any information indicating whether Wark suffered broken bones in the fall, but said he did somehow injure a wrist.

Cianbro officials have begun an investigation into the accident, Hutchins said, and the U.S. Occupational Safety and Health Administration is involved. She believes other federal regulatory agencies also will be investigating. A Dragon official could not be reached late Friday for information on the investigation.