Is it possible to be legally arrested and not be aware of what crime you are being arrested for? In the UK being told what crime is a required part of the arrest procedure.
It is in the US as well. You have a Constitutional right to be informed of the charges being brought against you. But, as already stated, there is a difference between arrested and detained in our legal system. You can be detained by law enforcement without charges for a limited time before they either have to charge you or release you. That time period is generally 24 hours, but under special circumstances they can request to keep you detained without charges for up to 96 hours.