Dear Sir i have a question about cyber crime act , my uncle who lives in NY USA , he and his son was …

Dear Sir i have a question about cyber crime act , my uncle who lives in NY USA , he and his son was threaten by a person lives in pakistan using facebook messenger he threats them if they come pakistan he will kill them both and many other serious threats, kindly guide me what is correct forum to proceed against the culprit and what kinda information we should provide to concerned authority/investigation agency. Thanks