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Mr Cruel victim 'wasn't going willingly'

The reward over the "Mr Cruel" murder of 13-year-old Karmein Chan is set to rise to $1 million. (AAP) Source: AAP

The last victim of 'Mr Cruel' whose crimes terrorised Melbourne families in the 1980s and 1990s may have been killed because she identified her captor.

Melbourne schoolgirl Karmein Chan promised to put up a fight if grabbed by 'Mr Cruel'.

The 13-year-old told her mum she wouldn't go willingly after seeing media reports about Mr Cruel's abduction of Nicola Lynas.

Karmein's comment proved to be prophetic: she was kidnapped by the child sex offender whose terrifying crimes culminated in her murder.

It may be that Karmein managed to identify Mr Cruel, who went to extremes to conceal his identity when he abducted 10-year-old Sharon Wills in 1988 and 13-year-old Nicola in 1990.

Mr Cruel told his victims his freedom was more important than their lives, Assistant Commissioner Steve Fontana said on the 25th anniversary of Karmein's abduction.

"We suspect she was pretty feisty and confronted and put up a fight with this particular offender and may have actually identified him," he said on Wednesday.

"We know that she was probably not a compliant type of victim in these matters."

It is speculation, for police still do not know who Mr Cruel is or why he stopped after Karmein.

He went to great lengths to ensure he was never identified by his many suspected victims, even when dropping off Nicola and Sharon.

"They had been thoroughly washed, clothes changed. There's a whole range of things that he did to make sure that they did not see him and that he did not try to leave any traces behind," Mr Fontana said.

Mr Fontana, a senior investigator in the original Mr Cruel task force, believes Mr Cruel is still alive and has kept reminders of his victims.

"We believe that he will still have items that were probably stolen from the children," he said.

Police had 32 `Sierra files' on people who could not be eliminated as possible suspects, one or two of whom were of "real interest".

The profile suggests someone well read, very intelligent and cautious, who meticulously planned and executed his crimes.

Mr Cruel may have died, moved or been jailed for other offences after taking Karmein, whose remains were found a year after her abduction.

He may have been one of the thousands of persons of interest interviewed by police.

"We may have actually spoken to the offender, which is why he hasn't offended again," Mr Fontana said.

Mr Cruel's horrific series of crimes terrified Melbourne families in the late 1980s and early 1990s, devastating Karmein's family and those of the other victims.

Karmein's family did not appear with Mr Fontana when he announced the reward over her murder has been increased to $1 million, because of the crime's continued significant impact on them.

"It was an emotional roller coaster for them - and still is," Mr Fontana said.

It also took a toll on the 40-strong Spectrum task force and second investigative team that reviewed its work and new leads earlier this decade.

"Everyone that's been involved in this case becomes very passionate about it," Mr Fontana said.