Similarly, while giving the employee involved in the altercation an opportunity to explain his side of things is a necessary formality, I would not expect this step to reveal any information that is useful or helpful to understand the situation. I would actually place much greater trust in the information disclosed by coworkers, especially those who were not directly involved in any specific incidents but who work closely enough with the individuals involved to shed a more objective light on what happened and what factors or behaviors they have observed that contributed to the problem. In that investigation, I would stress that their input would remain confidential to ensure their cooperation without fear of reprisal from the individual at issue.
Response to Aneka
With respect to the lateness issue, I would be much more interested in the information from the interview with the manager about how clearly he has explained scheduling and lateness policies than in the excuses of an employee who has been regularly coming in late. As far as the disciplinary issues are concerned, I like the anger control training idea and I would make it a condition of continued employment.
In that regard, the fact that the employee involved is an at-will employee is very helpful since, technically, he could be terminated without cause. However, because of the potential unemployment claim financial liability, I would certainly document the entire series of events to establish that there was cause for termination in the event that remedial steps fail to rectify the situation..