The force was so concerned about the danger he posed to women that they built a world around him populated by police officers posing as his friends plus their girlfriends and family.
Titled 'Operation Misty' the undercover sting paid dividends when Allen confessed to the murder of Samantha Class in 1997, years after being acquitted by a jury.
The recording of Allen's confession to the officer, named only as Ian in court, was crucial when the Director of Public Prosecutions successfully applied to the Court of Appeal to quash the not guilty verdict reached by a jury in 2000.
It was also a central plank of the case that led to his eventual conviction.
The jury heard that Operation Misty began in 2010 when Allen returned to live in the Humberside Police area after being released from prison.
Allen was recorded in December 2010 telling Ian that a sex worker had threatened to report him for rape if he did not pay her "so I strangled her ... and dumped her in the Humber".
Prosecutors said this was the "clearest possible admission to the defendant's guilt in murdering Samantha Class".
Detective Chief Inspector Christine Calvert, who led the recent investigation for Humberside Police, was not involved in the undercover operation a decade ago but said: "It would have been a huge decision."
She added: "It was a two-year operation and they used seven different undercover officers.
"They all played varied parts in the operation."
She said Ian did "a fantastic job and deserves every commendation going".
Asked why Humberside Police were so determined to put Allen behind bars despite his 2000 acquittal, Ms Calvert said: "I think it was the risk he posed to the public over that period of time.
"It was just getting justice, really, for the family. Samantha's daughter was only 10 when it happened.
"All three of Samantha's children have grown up without a mother and it was getting that justice for them, really."