I own and operate a web-based forum. Users can post information for other users to view. While I am not a mandated reporter as defined by the state I live in, the site users come from different parts of the United States and some from other countries. Would I be obligated to report a crime or suspected abuse which is posted by a user to law-enforcement agencies? If so, what criteria should be used to determine which agency or agencies I report to?
My recommendation is if you know the general location of the user, find them through those means... otherwise contact local law enforcement and explain the problem. In the United States, credible communications of an intended crime rise to the FBI as they will likely be interstate matters (If you're in Texas and being threatened by a crime by someone from spins the spinner Rhode Island and it's legitimate enough threat, than the crime crosses State Lines and is in Federal Jurisdiction.).
As the owner/operator, you have an incredible amount of power to figure out identifiable information that can help you. Speaking from personal excperience, I once had a web-forum user who was threatening suicide in one of our chat rooms. I was able to check on forum for the e-mail account and was able to link that through a google search of the address to a social-media account that included a lot of discussion on topics our forum was set up for (Star Trek fans) AND some hobbies I knew the guy was involved with... as well as a name and some other region specific affiliations... all that was given to the suicide prevention hotline worker I was talking too and forwarded to the local police... and moments later in chat, the suicidal individual typed up "OMG, who called the cops."
Suffice to say, it's not hard in the social media era for a determined individual to find someone... You just need to access the right information, and rely on the fact that most people use the same passwords for e-mail registrations.