Failures by Devon and Cornwall police meant historic rape cases went unsolved for two decades, the police watchdog said today.
The Independent Police Complaints Commission (IPCC) said the force "missed opportunities" to bring a conviction against rapist Shaun Harrison after it failed to upload DNA samples to a national database.
A DNA sample from a 16-year-old girl who was raped in Plymouth in 1989 was not put on the National DNA Database when it came into existence in 1995.
It meant that Mr Harrison was not linked to the attack when a DNA sample was taken from him following an arrest for drink driving in 2000.
It was only in August 2010 when the Forensic Science Service pointed out that the sample had not been uploaded that Mr Harrison was connected with the crime.
He pleaded guilty and was sentenced to eight years in custody for the 1989 rape, and a further four years for a second rape in 1994.
A complete review into unsolved sexual abuse cases with retained forensic material, including three cases from 1989, 1990 and 1993, was launched.
As a result Alexander Shepherd was jailed for six years and four months after a 27-year-old woman was violently assaulted and raped in Plymouth in 1990, while William Poad was given a six-year sentence for an indecent assault on an 11-year-old girl in Penzance, Cornwall, in 1993, the IPCC said.
The third case, involving the rape of a 15-year-old girl in St Mawes in Cornwall in 1989, "did not proceed further."