The investigation, code-named Operation Russell, allegedly raised questions about former commander Ray Adams's conduct before the Lawrence case, the Guardian said.
There was reportedly insufficient evidence to bring criminal charges and Adams told the newspaper this exonerates him and denied any wrongdoing.
More top news
A man has been rescued by firefighters - after getting his head stuck inside a washing machine.
Late-running passengers who attempt to pass through security with less than 30 minutes before their flight departs will now be stopped.
A Ukrainian pilot who spent two years in prison in Russia has been sworn into national parliament after being released last week.