Lewis Daynes, from Grays, Essex, was jailed for life for stabbing Breck Bednar from Surrey in February 2014.
In 2011, Daynes had been arrested over the suspected rape of another boy.
An investigation by the Independent Police Complaints Commission (IPCC) found no issue with the way Essex Police handled the rape inquiry.
Following an investigation, the force decided to take no further action against Daynes for the 2011 allegation.
Detectives were found to have "actively pursued all lines of enquiry based on the information available" and the actions taken were "justified and proportionate in the circumstances", the IPCC said.
On Monday, the IPCC found a call from Breck's mother, Lorin LaFave, reporting a change in her son's behaviour should have alerted officers to the possibility he was being groomed.
Daynes, of Rosebery Road, Grays, was 19 when he was jailed in January this year.
He is currently serving a minimum term of 25 years for the murder, which Chelmsford Crown Court heard had a sexual motive.
A police force has been cleared of blame for the way it investigated a teenager who went on to murder a 14-year-old boy he met online.