Many organizations have well established teams with close interpersonal relationships. Investigating your own may cause bias to the analysis. Internal investigator may face social or economic or social pressure either from close colleagues or supervisors.
Concern of organization’s reputational risks may cause bias to the investigation. Investigator may be worried on how the outcomes will affect the colleagues, or themselves. Thus, it is unfair to expose an employee to the situation of having to investigate his/her own colleagues or the team.
Whilst integrity is highly personal attribute, an external investigator faces less pressure on his/her objectivity. It is also easier to see the interrelations and correlation from an arm’s length. There is no bias from the familiarity of the organization, people or the situation.