Officials do not know how many keys were sold

The state spent $6,000 on refurbishing the Anamosa State Penitentiary after keys that belonged to a guard who retired in the 70s were auctioned.

Officials at the 135-year-old jail were not sure if the keys still worked but could not take any chances, a warden told KCCI-TV in Des Moines.

Anamosa is a medium-security jail but holds a number of violent criminals.

'Never at risk'

Warden Jerry Burt told the TV station many of the cell blocks from that time were still in use and the keys might fit.

"We do know there were a number of keys and it's a possibility," he said.

"We don't know exactly how many keys were sold. We don't know where they went... We don't have any information that says any particular key that fit any particular lock may have been compromised."

The warden said the public had never been at risk but the locks were changed as a precaution.

The keys were bought at a sale following the death of the prison guard. They were advertised as once having opened the jail's locks.

Anamosa is about 25 miles (40km) north-east of the town of Cedar Rapids and houses more than 1,000 inmates.

The Anamosa Penitentiary Museum lists the jail's worst inmate as John Wayne Gacy.

He spent two years there from 1968 and helped build a miniature golf course, but after his release went on to kill 33 men before being executed in Illinois in 1994.