Detectives are trying to trace more than 60 men they believe were victims of Reynhard Sinaga, the man described as Britain's most prolific rapist.
Sinaga, 38, is serving a minimum of 40 years in jail after being convicted of committing 159 offences, including 136 rapes, against dozens of men in Manchester.
The Indonesia-born student was jailed for offences against 48 different men but police believe his total number of victims to be 206.
Greater Manchester police said on Thursday that it was still trying to trace 60 more men, so far unidentified, believed to have been abused by Sinaga, who pretended to be a good Samaritan to lure often-drunk young men to his flat from outside city centre nightclubs.
The force said 155 people had come forward to report further alleged sexual offences by Sinaga after it issued a public appeal in January last year.
Police said on Thursday that 23 of those have now been confirmed as potential victims but had decided not to pursue a further prosecution against Sinaga, who judge Suzanne Goddard QC said last year would never be safe to be released from prison.
DCI Zed Ali, who is leading the investigation, said: "Supporting those affected by Sinaga's despicable crimes has always been our priority, alongside securing the maximum possible jail term for him.
"Our appeal, launched last year, has so far identified 23 further victims of Sinaga. I commend their bravery for getting in touch and the strength and dignity they have shown throughout the past year during our further investigations.
"Ultimately, they decided not to pursue further prosecutions given Sinaga is already serving a record sentence for rape offences, and it will be four decades before he is even considered for release.
"That they are satisfied justice has been done is testament to the hard work of our officers, who were able to secure the evidence needed to put Sinaga in prison - where he belongs."
He added: "I would urge anyone who thinks they may have been a victim to please get in touch with us.
"Specialist staff involved in Operation Island remain key members of our force and will continue to investigate any further inquiries that come to light.
"If you don't feel comfortable speaking to police, specialist support services are widely available with organisations such as St Mary's SARC and Survivors Manchester."