Seeing a lot of responses that are wrong because they are talking about what police in the US can do. This article is about border crossing where border patrol can ask you to unlock the phone without any warrant/etc. If you refuse then you can be denied entry to the country (although I believe that is just non-citizens). Not sure if things can escalate from there.

Edit: which means if your phone was off, they’d just ask you to turn it on and unlock it.