Would you mind terribly quoting the law that requires me to submit to the physical act of unlocking my phone? Since it's not a two-factor system, it is equivalent to a passphrase, which (IIRC) has been upheld to be a form of testimony, and therefore the fifth amendment SHOULD allow the suspect to refuse to "utter their fingerprint", so to speak.
In any case, if the fingerprint isn't the same "testimony" as a passphrase, the suspect is not in violation of the law after 48 hours has elapsed, since "uttering their fingerprint" (as I'm choosing to style it) after 48 hours wouldn't unlock the phone any more anyway.
Providing a fingerprint isn't considered testimony, and the law doesn't currently make an exception for a fingerprint used as the equivalent of a passphrase. The fifth amendment protects knowledge, but not physical constructs like a fingerprint or DNA swab[1].
So, you can be compelled to provide a fingerprint, and the use that the fingerprint is put to is outside the scope of 5th amendment protections.
It's gonna get really interesting if/when neuroscience advances to the point where we can just read a passcode out of someone's brainwaves. I don't see any legal difference between the fingerprint case and the brain-scan case (both are measuring a physical aspect of the person) but it's going to completely bypass the fifth amendment.
Merriam-Webster defines "testimony" as "something that someone says especially in a court of law while formally promising to tell the truth." The Supreme Court has interpreted this very broadly, to include anything that requires accessing information from the mind. That's why some courts have held that passwords are testimonial. But to read it to include touching your thumb to a fingerprint sensor obliterates any meaning the word has.
The reason the courts find the production of passwords to be testimonial is that they are used to access potentially incriminating evidence the specific existence of which the prosecution is not already aware of.
Further, the Supreme Court Justice Clarence Thomas (with Scalia joining) said that the 5th amendment applies to, "compelled production not just of incriminating testimony, but of any incriminating evidence." https://www.law.cornell.edu/supct/html/99-166.ZC.html
The courts' definition of "witness" is what's relevant in addition to "testimony". Is your smartphone a privileged "witness"? I would think that to be the ultimate decision that needs to be made by the Supreme Court, and one that may in fact hinge on what person is next appointed to the Supreme Court.
In any case, if the fingerprint isn't the same "testimony" as a passphrase, the suspect is not in violation of the law after 48 hours has elapsed, since "uttering their fingerprint" (as I'm choosing to style it) after 48 hours wouldn't unlock the phone any more anyway.