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If the employee submitted a written complaint, review that carefully and then have a meeting with them to talk about the incident or issue they brought to your attention. Let the employee know that the company is taking their complaint seriously and will thoroughly investigate the situation. Remind them about laws that prohibit retaliation for filing complaints, and tell them to come to you if they get any blowback as a result of their complaint.  You also should assure the employee that their statements will be kept confidential to every extent possible, and that their privacy will be respected and protected throughout the investigation. Make sure you get as much information and facts about the situation from the employee as you can. If they have any documents or other physical evidence regarding the incident or issue, get copies for the official record. In some situations, you need to take action immediately, either to protect the safety or privacy of the employee who filed the complaint, or to protect the integrity of the investigation.  For example, if an employee alleges their immediate supervisor is sexually harassing them, you will need to separate those two employees so that the accused supervisor is no longer directly supervising the employee who complained. Take care not to take actions that could be construed as punitive or disciplinary in nature. For example, if you need to remove an employee from the workplace while the investigation is being conducted, they should have time off with pay. Based on the written complaint and your conversation with the employee who complained, look at the employee handbook. Refresh your knowledge of the policies or rules the employee is alleged to have violated. If you notice any gray areas in a policy, or ways in which the policy could be construed differently than it was intended, make a note of them so you can bring it up and have it clarified later. The purpose and scope of the investigation should be carefully delineated from the outset to keep your investigation on track. Keep your focus as narrow as possible based on the incident or issue alleged in the employee's complaint. Be as narrow and specific as possible. For example, if you have an employee who alleges their immediate supervisor sexually harassed them, your focus should be solely on the interaction between these two employees. If additional behavior is uncovered, make it the subject of a separate investigation rather than broadening the scope of the original investigation. You know you'll need to interview the accused employee, and you may need to have another, more formal interviewer with the employee who filed the complaint. Based on your initial conversation with that employee, you should have the names of potential witnesses to interview as well. You may want to talk to other employees as well, based on the accused employee's position and anyone who may have had supervisory authority over the accused employee. For example, if the accused employee was the complaining employee's immediate supervisor, you may want to interview the accused employee's direct report. Before the interviews take place, sketch out the basic information you want to learn from each of the people you interview. Many of these questions will be similar, but some may be different depending on the employee's position and their role in the incident.  For example, if you're interviewing a witness, include questions about what they saw or heard. You also want to find out how well they know the employees involved in the situation. Ask each employee if they've talked to any other employees about the situation, so you can interview them as well. Many complaints come down to one employee's word against another. The more physical evidence you have, the better able you are to get to the bottom of the incident.  Many employees aren't going to be thinking in terms of gathering evidence when a situation begins, or even after it escalates. You may have to think outside the box to find the evidence you need. For example, if the accused employee claimed they were making copies when the alleged incident occurred, you might check the log on the copy machine to see if it was being used at the time of the incident. Looking at the employees' personal files will give you better context into the background of the employees. Just be careful to remain objective. Don't let information in the employees' personnel files color your opinion of the situation before the investigation begins.  For example, you may find that the employee who filed the complaint has filed several complaints previously regarding other employees. This could mean any number of things, but it doesn't mean you should take the complaint any less seriously. On the other hand, you may find that the employee accused of misconduct has previously been accused of similar behavior by other employees. One of the most important factors in a successful workplace investigation is who conducts the interviews and manages the investigation as a whole. While the person should have authority, they also need to be able to put people at ease. The employees interviewed should feel comfortable opening up to them. If the situation is serious, or if a lawsuit seems likely, you typically want to hire an outside independent investigator to conduct the interviews on behalf of the company and report back to the executives or managers. Many employee complaints can quickly devolve into a case of "he said, she said" where you find it impossible to uncover the truth. Getting as much physical evidence as you can before you start talking to people can help you organize your questions and put the investigation in perspective.  Certain physical evidence, such as machine logs, call logs, or company emails, can be gathered without alerting anyone that an investigation is taking place. At this stage, try to keep your evidence collection confidential to avoid alerting anyone that there might be an issue. In some cases you may need additional expertise to assist with a thorough investigation. While you may not know this until you get into the thick of the investigation, try to determine during the planning stages if it's possible additional expertise will be required. If you're not sure whether expert opinions might be needed, you might want to talk to an employment law attorney before you begin the investigation. This is especially advisable if it seems a lawsuit may result from the investigation if the complaint isn't handled to the complaining employee's satisfaction.

Summary:
Talk to the employee who complained. Act immediately if the situation needs to be contained. Review applicable company policies and procedures. Define the purpose and scope of the investigation clearly. Make a list of people to interview. Draft a list of questions. Determine what evidence you need. Review the personnel files of all employees involved. Choose a skilled interviewer with a pleasant demeanor. Gather physical evidence before conducting interviews. Line up experts if necessary.