There have been riots in said town that have gotten so bad they've outstripped the police's ability to deal with them, and while the National Guard has been called in, it's feared that they won't be able to handle it either, so a small force from the 75th Ranger Regiment is being held in reserve but ready for action nearby.
I know about the Posse Comitatus Act (http://en.wikipedia.org/wiki/Posse_Comitatus_Act) which forbids the deployment of the Army within the US to carry out the law, but one of the expressly stated exceptions is "support roles under the Joint Special Operations Command."
(Additionally, though this is not public knowledge, it's suspected that those responsible for the riots are part of a terrorist cell, and this is also one of the few expressly permitted exceptions to the Act as of 2012 - http://en.wikipedia.org/wiki/National_Defense_Authorization_Act_for_Fiscal_Year_2012 - and forces under JSOC are there to move in if this is confirmed.)
All of that being said, the scenario is this: said FBI agent is on a case near said town where the Rangers are waiting, and recognizes one of the Rangers, because he's his cousin. Would they be allowed to talk? If so, what information would the soldier be allowed to give him? (I'm assuming "zero" WRT the operation, but it's pretty obvious what they're doing there.) It's been over a day since the riots started and the soldier hasn't seen any action yet. Lastly, would the FBI agent be permitted to speak to the soldier's commanding officer if he had information that might be relevant to them that he had recently acquired, or would he have to go through some other, official channel? If it matters: his cousin is a corporal, the officer overseeing the entire operation here is a major, and the FBI agent is a long-time veteran somewhat renowned within the Bureau and other law-enforcement.
I don't really know where to search for this because it's my understanding that this has never happened before. US Marines were called in to help suppress the riots in LA in 1992, but I can't find anything on their interactions with (friendly) civilians during that time. I searched "military-civilian interactions" but the results (e.g. http://www.usma.edu/cnrcd/sitepages/Military%20Civilian%20Interaction.aspx) had to do with interactions on foreign soil. I searched for "joint FBI-military engagements" and found that the FBI does indeed work directly with JSOC when necessary (for reference: http://www.washingtonpost.com/world/national-security/inside-the-fbis-secret-relationship-with-the-militarys-special-operations/2014/04/10/dcca3460-be84-11e3-b195-dd0c1174052c_story.html) but again, these are all for operations on foreign soil.