Half a century after a notorious prison escape from Alcatraz Island in San Francisco Bay, the FBI has created replicas of decoy heads that inmates used to distract guards from a plan that still captivates researchers and tourists.

Authorities last week unveiled 3D-printed copies of the decoys that inmates Frank Morris and brothers John and Clarence Anglin constructed with soap, plaster, and human hair.

Morris and the Anglins  placed the imitations in their beds and climbed through a wall to escape the island prison, a mile and a quarter from land. The men were never found.

Inmate Clayton West also created a head but never made it out of the maximum-security prison that housed dangerous criminals and offenders with a history of escaping.

Authorities said they made replicas instead of displaying the original decoys because they are fragile and remain evidence in the still-open investigation into the escape by the U.S. Marshals Service.

"We understand the original items can't be out here — they've got to be archived," said John F. Bennett, FBI special agent in charge in San Francisco. "But we recognize that those items are also part of the rich and historic fabric and the  landmark of this city."

Bennett said a team from the FBI's laboratory in Quantico, Va., traveled to San Francisco to scan the original decoys. Employees at the lab donated their own hair to accurately re-create the original masks, which included hair that the inmates had collected from the prison barber shop.

"The hair and the paint on here is exactly what the prisoners did," Bennett said, showing the replicas brought to the island in black, waterproof cases.

The FBI investigated the prison break — which was featured in the 1979 movie Escape from Alcatraz starring Clint Eastwood — for 17 years before it was turned over to the Marshals Service.

Models of the heads that were used in the movie have been on display to Alcatraz visitors, and the FBI hopes the public will soon be able to view the agency's replicas, which were unveiled along with "Wanted" signs for the  long-escaped inmates.

Authorities continue to investigate all credible leads, said Don O'Keefe, U.S. marshal for the Northern District of California.

"Some may believe that we're chasing shadows, but our efforts are meant not just to perform due diligence, but to be a warning to other fugitives, that U.S. Marshals don't give up because of the passing of time," he said in a statement.