I'm not aware of any legal precedent under which you can refuse to cooperate with an ongoing police investigation, except where such investigation would violate

I'm not aware of any legal precedent under which you can refuse to cooperate with an ongoing police investigation, except where such investigation would violate your basic rights (e.g. Bill of Rights in the USA) In the USA you'd probably have to contend it is either unreasonable search and/or seizure, which would be a difficult case to make. Edit: Looks like the SCOTUS ruled on DNA testing & the Fourth Amendment just last year.