Conducting proper investigation is important not only to the parties involved in that conflict so that their issue is resolved, but also to the employer. This is because if the investigation is not conducted correctly, the alleged victim of harassment can use the substandard quality of the investigation in his claims against the employer, showing that the employer is that bad guy because they didn't care to do the investigation correctly, when they could and should have.
Here are two board and critically important principals of every proper workplace investigation:
1. The investigation has to be impartial or as impartial as possible. This means that ideally, the investigator must not be an internal employee, but someone who is hired from the outside. There are plenty of companies out there who offer quality workplace investigators, who can come in and do a good job interviewing all the parties involved and witnesses, and issuing reasonable findings for you, the employer, to work with. Ideally, an employer should be hinting to the investigator at the findings they would like to see and let the investigator do their job completely independently.
2. All relevant witnesses should be interviewed. There are almost always two sides to every story. To get the most complete picture of the situation and the most reliable findings, the investigator should talk to every witness of the relevant events on both sides - the alleged victim and the accused. An employer should not be preventing the investigator from doing as good and as complete of a job as they would like to, if they want to get to the bottom of what happened in any given dispute.
Sticking to the above basic principles alone should help any employer make sure that their investigation is conducted correctly, and it will not backfire or any of the parties involved initiates litigation.