Cannan, jailed in 1989 for a series of horrific attacks on young professional women, died in his prison cell aged 70 last month.
He was named by cops as the chief suspect in one of Britain's most notorious unsolved crimes, Suzy's 1986 abduction and presumed murder.
Cannan always denied involvement but his own family have said he was a manipulative criminal who revelled in his crimes.
A family member - who we agreed not to name - said: "On my last visit [to prison] with my mother in 2000, I asked him to own up but I'm afraid he hadn't got it in him to come clean.
"We are 99 per cent sure he was involved in this poor girl's abduction.
"I'm sure John delighted in taking his secrets to the grave and gave him a feeling of perverted power.
"He was always bereft of responsibility, humility and engulfed in his own deluded selfishness and importance."
Cannan was sentenced to life for abducting newlywed Shirley Banks in Bristol in 1987 and bludgeoning her to death with a rock. He dumped her naked body at Dead Woman's Ditch in Somerset's Quantock Hills.
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