The IOPC is examining inquiries from 1991, 2001 and 2006 and said it was considering the conduct and actions of 13 individuals, though none are serving officers.
It has sent an update to the Independent Inquiry into Child Sexual Abuse (IICSA).
IICSA has received complaints from more than 30 people alleging the former Labour MP abused victims between the 1950s and 1980s.
While emphasising its investigation was ongoing, the IOPC outlined “matters of concern” including:
In the early 1990s, before a formal investigation into Lord Janner commenced, police records indicate “a number of references to a relationship, including a sexual relationship, between Lord Janner and a child”, but there is no evidence the claims were looked into
Once an investigation did begin, lines of inquiry “appear from the evidence not to have been carried out” and “there is an indication that senior officers may have influenced decisions regarding the inquiries being carried out”
In 2001-2002 “allegations made by former children’s home residents appear from the evidence available not to have been investigated or recorded” and “documented results of investigative actions regarding Lord Janner appear to contain information that could be interpreted as misleading and/or inaccurate”
The update said a new referral was made to the IOPC in February, which “based on the evidence reviewed” indicated “police documents may have been inappropriately modified”.
The IOPC said all those under investigation had been issued with notices regarding potential criminal offences and potential gross misconduct.
It said hoped to produce a final report by the end of June.
IICSA said it had paused its work regarding Lord Janner to avoid any duplication.
Lord Janner, who was an MP in Leicester for nearly 30 years, died shortly after a judge had ruled he was not fit to stand trial for alleged child sex offences.
His son Daniel Janner QC has consistently criticised all investigations into abuse allegations and said the IOPC update “should never have been published”.