Hostile Work environment

I received a package at my job with feces and a note inside of the manila envelope. I recognized the handwriting on the outside of the package as belonging to a co-worker and informed my employers. My employers called the Police and a complaint of aggravated harassment was made. There is a internal and Police investigation taking place. How can I prove the handwriting on the outside of the package belongs to my co-worker? Do I have a case against the company regarding a Hostile Working Environment??