Ok your employer does certainly have a duty to treat this matter seriously and investigate and try to find the author. This is a matter of gross misconduct and the responsible employee should be dismissed.
I do not think that this is a criminal matter, the police are likely to say that this is a civil matter to be dealt with by the employer.
You should now raise a grievance to ask your employer:
a) what measures it is taking to investigate the matter and to prevent this from happening in the future?
b) For reasurrance that the comments made in the letter are not reflective of the feelings of management.
You can state that you believe that the employer must positively respond to ensure:
- the health and safety of the employees
- the morale of employees is not unduely affected.
I do hope that this matter does resolve itself. It may be, however that as everyone is a 'suspect' the employer cannot give too much detail of how it is investigating.
I would be grateful if you would press ACCEPT if you have found my answer useful. I will then leave the question open to answer your follow on questions for free.