A police force allegedly returned a laptop containing images of abuse to a man who was later jailed for rape after an investigation by another force.
The Independent Office for Police Conduct (IOPC) said it was investigating Essex Police's handling of the investigation.
In a statement, the IOPC said the force had handed back computers that "may have contained more than 100 indecent images" to Paul Ashbury, 52, of King's Lynn, Norfolk.
Ashbury was arrested by Essex Police in 2012 on suspicion of sexual assault against a child and possessing indecent images, and officers seized computers from him.
These were returned to Ashbury in 2013 when they decided to take no further action against him.
Ashbury was arrested by Norfolk Police in 2017 and earlier this year was jailed for 15 years after admitting offences including rape, sexual assault, sexual activity with a child and making indecent photographs of a child, the IOPC said.
"It appears when Norfolk Police officers examined Ashbury's laptop they found additional child abuse images which it is believed had not been identified during Essex Police's investigation," the IOPC said in a statement.
The IOPC investigation will examine the conduct of officers from Essex Police's child abuse investigation team (CAIT) and whether the investigation into Ashbury was carried out in accordance with the force's policy, procedures and guidelines.
Sarah Green, IOPC regional director for the South East, said: "These are extremely serious allegations regarding how Essex Police investigated a man who later admitted multiple child sexual offences following the investigation by Norfolk Police.
"We will examine the officers' actions and what if anything they could have done differently."
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