How do I find out who the sender is on Facebook?

[Tessa]

I admin a page on Facebook and this girl messages us asking if we could share her page. Some other admin started sending her hate for no reason and didn't include their signature/sign off name, so we don't know who the sender is. Me and two other admins are working hard to find out who the sender is so we can get them kicked off the page. No one is confessing. How do I find out who this is so I can inform the owner? Is there a way I can trace the message back to a computer and get their profile? Is there anyway I can find out? I can't really report the message because then I will basically be reporting the page. Someone help?