He has a connection to Iran - wife and kids live there. Not as strong as saying he works for or is an agent of the gov’t there, though. The devil’s advocate might say that, “he’s got kids in Iran so Iran has the power to compel him to do this, and of course Iran isn’t going to issue him a gov’t ID that says gov't assassin recruiter if anything they’d try to distance themselves as much as possible to give themselves plausible deniability.”

Still, it’s reasonable to ask for evidence. I’d imagine more proof is forthcoming soon though - the FBI usually doesn’t comment on active investigations, which this would be (if the FBI is really investigating it, as opposed to the CIA or some other agency).