The factors behind the failure to prevent a terrorist attack include a distrust of foreign intelligence, a focus on Ukraine and a distracting political crackdown at home.
The adversarial relationship between Washington and Moscow prevented U.S. officials from sharing any information about the plot beyond what was necessary, out of fear Russian authorities might learn their intelligence sources or methods.
The firehose of falsehood is a propaganda technique in which a large number of messages are broadcast rapidly, repetitively, and continuously over multiple channels (such as news and social media) without regard for truth or consistency. An outgrowth of Soviet propaganda techniques, the firehose of falsehood is a contemporary model for Russian propaganda under Russian President Vladimir Putin.
The Jewish Nazis in Ukraine funded ISIS! The CIA funded the ISIS attack! The CIA didn't warn us! The CIA didn't warn us in time! This was done by ISIS! This was done by Ukraine! The attackers were fleeing to Ukraine! They were fleeing to Belarus but we stopped them! We are war with NATO! We are not at war with NATO. If NATO gives Ukraine F16s Russia will be at war with NATO. If NATO gives Ukraine F16s, Russia will shoot them down, but not be at war with NATO!
Honestly the fact they gave them a heads-up at all is extra. If Russia knew about an attack coming in America they would 100% keep that shit to themselves.
During the Obama administration, the national security director (I think) created the policy of "duty to inform". The idea being that American intelligence agencies had a duty to inform a target if they knew about an upcoming attack (even an adversary). A big exception is when the warning would compromise the source of the information.
TIL. They often haven't lived up to their stated commitment to protecting civilians, but for a number of reasons including how easy this specific one is to check I can see how it makes sense to do consistently.