I don't quite understand.
What, in the US, what legal -- as opposed to moral -- obligations does someone who attended the rally have?
Are they obliged to volunteer the information to the FBI that they were there, and to agree be interviewed if the FBI wishes, and to offer to allow agents to examine any phones or photographic equipment they had with them?
Or if the FBI turn up at their door, are they obliged to say anything other than, "I have nothing to say to you"?
That may well arouse the agents' suspicions they have something to hide, and lead to their temporary arrest while they're held for further questioning, but if they insist on standing upon their constitutional right to silence, and their protections against unlawful search and seizure, what offence are they committing by so doing?
I know that, in the movies and tv shows, cops often threaten uncooperative witnesses with arrest for "obstruction" or similar offences, but I've always assumed that's a convenient fictional device rather than the way US law actually works in practice.
In the UK, and probably in the US as well, there are -- contentiously, in some cases -- specific circumstances in which people in particular roles have to report suspicious behaviour to the police (e.g. child welfare cases, or teachers suspecting their students are being radicalised by terrorist groups) but those are exceptions, made by specific laws creating particular obligations to disclose particular matters in particular circumstances.
Assuming there's no general rule, and I don't think there is, in the US that you must answer any questions an FBI officer wishes to ask (though it might be both appropriate and prudent so to do in any particular circumstances), when is a potential witness legally obliged to cooperate with them, or even to make himself known to them?