I just checked fbi.gov and there is no press release announcing this. If this is truly information from the FBI, why do they not communicate it through official trusted channels?
1) The story is true but not meant for our eyes, 2) The story is true and meant for our eyes, but they can't officially say so or haven't yet prepared their statement, 3) The story is false and meant for our eyes as a form of propaganda, 4) ... other options
I think the best course of action is to treat the story as if it never happened. I'm utterly exhausted by what appears to be repeated propaganda via the US news media, covering their asses with this very tired "anonymous source" trick. If the source is anonymous it garners zero trust, and you just fool yourself if you think otherwise and choose to believe it because it is exciting/scary/wishful/etc... unless you trust these particular media sources (I don't). I'm fairly sure that most of the anonymous leaks during the Russia Collusion investigation turned out to be false propaganda.
If the government wants to warn us of something, they have to actually do that -- they can't play these games and presume we are going to react (if option 2) because we are exhausted by manipulation. I will take it seriously as soon as (and only if) the FBI takes communicating it seriously.
Anonymous to readers != anonymous to the journalist. The journalist can verify the source, and also guard his or her anonymity for their safety. This is common practice in journalism.
It's "common practice" because it gives unrestricted ability to fabricate any kind of narration without repercussions as nobody can verify it.