Officials at the Minnesota Correctional Facility in Stillwater say they found the beginning of a tunnel that was being dug in a basement area of one of the facility's buildings.
Corrections Commissioner Joan Fabian described the tunnel as "well-hidden and sophisticated."
The tunnel was discovered Wednesday around 11:30 a.m. by prison staff. Officials say the tunnel was found as part of its regular security program. They say the tunnel was about 50 feet from the prison wall.
Fabian praised the prison staff with thwarting what she called an apparent escape attempt.
Once the tunnel was discovered, inmates were ordered back to their cells. All of them were accounted for and officials say the public is not at risk.
The Department of Corrections' Office of Special Investigations is looking into the apparent escape attempt.
The Stillwater facility is a high-security prison which houses 1,400 inmates.
An inmate unsuccessfully tried to escape from Stillwater prison in 2005. The last successful escape from the facility was 1982, when two inmates hid in cardboard boxes that were loaded onto a truck.