Now he is to probe his former colleagues.
John Mitchell, the retired head of Strathclyde CID, has been chosed for a key post in Scotland's beefed-up police watchdog.
The new director of investigations, who will work under the Police Complaints Commissioner, Professor John McNeill, will play a crucial role in policing Scotland's police as eight territorial forces and other bodies merge next spring. Mr Mitchell retired as a detective chief superintendent in July after leading investigations as sensitive as the bomb threat to Celtic manager Neil Lennon and three other prominent Scottish Catholics.
He said: "I am delighted and honoured to have been asked to take up the role of director of investigations by the commissioner.
"My priority will be to ensure the investigations team is resourced, trained and ready to carry out effective investigations from April next year."
Until now Mr McNeill has mostly reviewed complaints about forces.
But, starting on April 1, he will have a new and more powerful post of Investigations and Review Commissioner or PIRC.
Under this beefed-up post, Mr McNeill will carry out a range of investigations in the way police work, including firearms incidents, deaths in custody, fatal crashes by police vehicles and criminality by the police.
Currently chief constables ask colleagues from other forces to investigate wrongdoing in their ranks but this situation will be impossible after the new Police Service of Scotland comes in to being on April 1.