Co-worker violence blamed on psychological disability: Can we ask for a medical certification?

Q. An employee of ours accused a co-worker of threatening him with physical harm. When we confronted the accused employee, he attributed the behavior to his psychological disorder and to a recent change in his prescription. We would like to verify the em­­ployee’s claims. Are we permitted to ask the em­­ployee for medical certification of his disability and a doctor’s statement regarding his prescribed medications?