The famous lecture by a law professor, “Don’t Talk to the Police”, also applies to the FBI
https://www.youtube.com/watch?v=d-7o9xYp7eE
If you are ever detained, your only response should be “Please direct any questions you might have to my attorney. Here’s his number.”
If they ask “May we see your phone?”, or any other statement phrased as a request, recognize that if you comply, then you have voluntarily handed over evidence. Your response should be “No, I do not give you permission to examine my phone/car/whatever. I wish to leave now. Am I free to go?”
Never volunteer any information. Never give permission, or say anything which could be interpreted as giving permission. If they have a warrant, they will TELL you to hand it over, not ask.
CAN NOT BE REPEATED TOO MANY TIMES!