What if a worker demands to know the reason for their termination?

Q: “We fired an employee without giving a reason. The employee is requesting the reason in writing. Do we have to comply? Also, we just learned that the new supervisor of the employee never met with the employee to discuss the employee’s position description or the supervisor expectations. The supervisor issued the employee a corrective action. In addition, the employee filed a complaint against someone in senior management. The complaint was never investigated. The employee was terminated the next day after filing the complaint. Is there any liability on the agency?” - Anonymous, Virginia