Keep screenshots of the post and keep archived copies at archive.is then report it to the police and to the media as well. You can also use the [Report] button

Keep screenshots of the post and keep archived copies at archive.is then report it to the police and to the media as well. You can also use the [Report] button of the offending post to see if FB's AI will remove it. The police can always request for the copy of the removed post from FB via https://facebook.com/records https://facebook.com/records