Not legal advice but I'm a US citizen and when I've refused to answer their intrusive questions the worst that has happened to me is they imprisoned me, stripped me naked and searched me, got a fraudulent warrant for an internal body search, and then sent me the bill after dumping me at the border when they found nothing.
They claimed a dog alerted on me. An anonymous dog, handled by an unnamed officer in the affidavit, which was used as 3rd party inter-species hearsay via a HSI officer to the assistant attorney and judge.
A dog did not alert on me. In fact it is against CBP policy to use dogs on a person, they are to be used on your articles.
Enjoy.