It wasn't exactly "The Shawshank Redemption" or even "Prison Break," but authorities said a serious escape attempt at the Stillwater prison was thwarted when a staff member discovered the beginnings of a "well-hidden sophisticated underground tunnel" in the basement of one of the prison's industry buildings.

There are no surveillance cameras in the area where the tunnel was found Wednesday morning, officials said Thursday. Four inmates work in that area under the supervision of one corrections officer. The area is used for inventory and storage of raw materials for other prison industries.

Warden Lynn Dingle said those four inmates are being questioned, but she would not say they were suspects in the escape attempt or identify them or the crimes that put them in prison.

"We found a variety of different instruments ... in the area of the discovery of the tunnel," Dingle said. She would not elaborate further, saying many of the details are classified because of the "potentially criminal" investigation.

The prison includes a walled perimeter around the medium and maximum-security areas, which includes the prison industry buildings.

The tunnel was discovered about 11:30 a.m. Wednesday by a supervisor, "who was doing what he was supposed to be doing when he was supposed to be doing it," said David Crist, an assistant commissioner in the Minnesota Department of Corrections.

Officials said all inmates immediately were sent back to their cells.

A head count showed no inmates were missing.

Was the tunnel hidden in a nook or cranny or was something used to cover it up? "Why don't we settle with a little of each," said Deputy Corrections Commissioner Dennis Benson.

Benson said the tunnel was "large enough for someone to actually enter," but he wouldn't give its dimensions or say what was used to construct it. He said the distance between the start of the tunnel and the perimeter wall was about 50 to 60 feet.

"They had quite a distance to go to get to the perimeter wall," he said.

Officials said this is the first tunnel ever discovered at a Minnesota prison but not the first in the nation.

Corrections Commissioner Joan Fabian said she wanted "to assure the public no one escaped and they are not in any kind of danger as a result of this."

Stillwater prison is a high-security facility with about 1,400 inmates and about 65 dayside corrections officers. About 350 inmates work six-hour days in the prison industries, which include manufacturing, paint shop, welding and machine shops and carpentry. There are 12 manufacturing supervisors who oversee the inmates.

The industry area is closed to inmates while an investigation continues into the escape attempt, authorities said.

Several inmates have walked away from the prison's minimum security grounds, but few have escaped from the medium- and maximum-security perimeter. In 1988, an inmate escaped by scaling the wall but was caught a half-block away.

In 1982, two inmates hid in large boxes and escaped in a mail truck. They were on the lam for some time, officials said Thursday.

Pat Pheifer • 651-298-1551