Iowa prison officials say the lockdown at the Fort Madison Prison began after a staffer smuggled in contraband.
Fred Scaletta, executive officer for the Iowa Department of Corrections, says all 560 inmates in the prison's maximum-security unit have been confined to their cells since March 12.
Officials say the security breech is under investigation and the staffer is on administrative leave.
The unit holds about 560 prisoners, many serving life sentences for rape and murder.
The lockdown did not affect inmates in the medium-security unit or the clinical unit.
Staff and inmates are safe and there are no reports of violent incidents.
A construction project is under way to build a new 800-bed maximum-security prison. It will open in January 2014.