A police constable found enough intelligence to conclude that Ian Huntley was a "serial sex attacker" three years before he murdered Jessica Chapman and Holly Wells.
PC Michael Harding, the only officer to compile an intelligence file on Huntley, said that he was "surprised" to discover that his report was deleted by the police a year after it was written.
The constable was giving evidence at a public inquiry into the failure of police vetting that allowed Huntley to get a job as a school caretaker despite being accused of a series of sex assaults, including four rapes and an assault on an 11-year-old girl.
The inquiry also heard evidence yesterday from the former head of a child protection unit at Humberside Police who admitted he had little idea how the force's intelligence databases worked.
PC Harding also disclosed that he did not realise that his intelligence report on Huntley could be deleted or "weeded" out by civilian staff.
Although Huntley had been attracting police attention since 1995, PC Harding was the first and only officer in the Humberside force to link the cases.
He told the Bichard inquiry, chaired by Sir Michael Bichard, that he took five or six hours looking through paper records in what he described as a "broom cupboard office" to find enough intelligence to conclude that Huntley is a "serial sex attacker and is at liberty to continue his activities".
PC Harding noticed the pattern in Huntley's activities while investigating an allegation of rape in July 1999.
That case ended in no action against Huntley but the officer filed his intelligence report - which listing four rape allegations made in the previous 12 months and one of indecent assault - and it was sent to the force's intelligence bureau.
However, the intelligence report was deleted from Humberside's database in July 2000 as part of a "weeding" process.
When asked if he thought it would be retained indefinitely, the officer said: "Yes, you would not put it in if it was going to be deleted."
Earlier the inquiry heard from former detective inspector Peter Billam, who for four years was in charge of the child protection unit in Grimsby before retiring after 30 years in Humberside Police in August 1997.
Mr Billam, who took no action against Huntley over three allegations of underage sex, admitted his decisions would have been "totally different" if he had linked all of the accusations.
The inquiry heard that even when a "disturbing picture" began to emerge about Huntley's activities, Mr Billam was either unaware of or unable to link all of the allegations against him.
He admitted he did not know what records were being retained, what intelligence was being passed on for use in the force's databases, and how the different recording systems worked.
He argued: "I didn't feel it was my responsibility."
The former officer said that he believed that once the intelligence and reports were compiled that they would be entered into one of three databases by police staff.
James Eadie, counsel for the inquiry, asked the officer: "Didn't you ever consider at any time you were heading up the child protection units, whether or not your information recording system, either manual or computerised, needed to be reviewed, whether they were adequate?"
Mr Billam replied: "No."
The flaws in the vetting procedures and police intelligence handling allowed Huntley to get his job at Soham Village College. It was while at the Cambridgeshire school that Huntley abducted and murdered Holly and Jessica in August 2002.
Huntley, 30, was jailed for life in December last year for the murders of the two 10-year-old girls.
- More about: