Completing the investigation is down to the user who is responsible for that area

Step 1: The user who has to complete the investigation will get an email notification and a notification in their task manager like this

Or they can go to Received Customer Complaints in the Customer Complaints Manager and see that a complaint requires their action

Step 2: Then select Respond

Step 3: Fill in details then select Save & Save Progress

Step 4: The status stays at Requires Your Attention as it needs to be sent