Originally Posted by Mookster
A warrant must be issued prior to search and seizure, as per the fourth amendment. You're wrong in two assertions: they do not have a legal right to your fingerprint or DNA sample upon entering custody and they cannot do whatever they please with it. That fingerprint is protected by your fourth amendment right. There is some "new" conflicting legislature on this, but it still states that if you refuse they need to get approval from a court. Your constitutional rights are supreme.
The fourth also protects you from search and seizure of your effects, and should surely protect your phone considering it explicitly states that they cannot take your "papers." If they want to take your phone and your fingerprints, they need a warrant for both.
DNA is a different story. It is not a means to ID you. Police will need a warrant for DNA.
Yes, they need a warrant for the phone. That is not in-dispute in the op. Once they have the warrant, can they use the fingerprints they already have to unlock your phone? Yes, they can. Just like they can hack or guess your password to your computer after the get a warrant for it.
Once you are in custody, they have the right to ID you. Sorry. And they can run that ID against anything they want. If you refuse to give your finger prints. Three things will happen.
1.) You will be held until you are force in front of a judge. The judge will order you to give up your finger prints. There has never been a case in which a judge has denied the police the right to take fingerprints.
2.) You will then be charged with resisting arrest and obstruction of justice.
3.) Failure to comply with the judges order will get you fines and jail time until you do so.
You ready for the kick in the head. Lets say that you have a key lockbox. The police find the key in your hand. They can use that key to unlock the box. Just like the can use your fingerprint, right next to the key, to unlock your phone.