In the US, if the police want to get a warrant to force someone to give a DNA sample in a murder investigation, is it always required that they must already have a suspect DNA sample to compare it to (i.e. collected from the scene, or believed to be connected with the crime somehow)?
I'll give a concrete example: suppose there's a murder, and the police don't have a DNA sample from the scene that they think could be the suspect. But they might get one some day - technology is always improving and there are clothes, objects etc. that might be enough, years from now. But it's speculative. Now suppose someone is named as a person of interest. Is it possible for investigators to get a warrant for that persons DNA? Or would the fact that there's currently nothing to compare it to make it impossible to get the warrant?
(I know that in some places they could take DNA if they arrest the person, but in this case I'm just asking about getting a warrant for DNA or "fluid samples" or whatever the term is. Let's say they don't have enough to arrest the person.)