Thing one, for the guys here to give you effective help, you need to supply more technical information about your video source. Is it analogue (VHS tape?) or is it digital. And what is the format of the material (NTSC or PAL, interlaced or progressive, MPEG2 or something else).
Thing two. While I am no expert in this field, I would imagine that if this is for use for legal purposes (e.g. to provide evidence that could potentially ultimately be used in court), you might need to fulfil certain legal criteria related to the preservation of evidence. The use of specialist forensic software designed to provide something like a technical audit trail of some kind. Or use some kind of legally approved forensics service or leave it to the police. As I say, I know nothing about this, but I would advise you be really, really careful. Perhaps you should consult your lawyer to understand the proper legal implications and any requirements to need to fulfil.