I like the British description: “assisting the police in their inquiries”.
Interestingly,, the DOJ and federal courts have no difinition for the term “person of interest”. It is simply a euphemism for “Suspect”,, and it’s catchy sounding for PIOs. It sounds like something an actress on CSI might say.
Senator Grassley even asked the Ashcroft DOJ to define the term in reference to it’s use in the 9/11 anthrax case, and after the response, concluded that it actually HAS no official definition.
Now the Brits,, thats an odd world with some very different approaches!