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Q: Trevor Gray was jailed in May 2012 for attacking a woman in her home, but then cleared last month after a retrial.
The Independent Police Complaints Commission (IPCC) said it was examining whether steps were taken to identify and locate significant witnesses.
Nottinghamshire Police said it "fully supported" the IPCC's investigation.
In 2012, Mr Gray, 49, was found guilty of rape, attempted rape and sexual touching. He was given an eight-year jail sentence.
Mr Gray appealed against the decision on the grounds police had failed to locate a significant witness who would have assisted his defence.
At the appeal hearing, evidence from a witness traced by Mr Gray's wife Alison led to the conviction being quashed.
A retrial was ordered at which Mr Gray was found not guilty of all charges.
Mr Gray said he found it difficult to understand why the witness had been overlooked by police.
IPCC Commissioner Derrick Campbell said: "Where justice has not been properly served, there are many victims.
"Our investigation will look at whether Nottinghamshire Police properly and thoroughly followed all lines of inquiry into this matter.
"It will also look at whether there are lessons for Nottinghamshire Police to learn from these events."
The IPCC added it would also look at what information was given to the Crown Prosecution Service.
The Police Federation, which represents rank and file officers, condemned the original investigation.
Phil Matthews, from the Federation, had previously said it "beggars belief" that a vital witness who was not traced by police was found by Mr Gray's wife in four days.
Mr Gray spent 14 months in prison and was dismissed from the force.
His appealed against his dismissal is still being considered by Nottinghamshire Police.
A:
Nottinghamshire Police's handling of a rape inquiry which led to one of its detectives being wrongly convicted is to be investigated.