I'm a big fan of the "random individual sent in time" scenarios. In this one imagine someone from today is sent back into the body of their childhood self. He appears in his body the first day of school the year that 9/11 occured, so around september 1st. He was not planning to travel and didn't have time to research or prepare.
Lets say further that he worked in the NSA as an adult, nothing high, but a low level analyst, programmer, translator whatever. Enough to know some of their general policies, and particular to have memorized the phone number to his office/cubicle, but not enough to have any deep insider knowledge that he could use to 'prove' he was in a position of knowledge or authority.
He calls up his old work number from an anonymous pay phone and warns them about 9/11. He won't get anyone with authority to do something directly, but surely the threat will be escalated up to important people quickly. He shares what an average individual may remember from the news; the targets hit, the terrorist home country and terrorist group, that they used razor blades and learned to fly but not land etc. However, he doesn't remember the exact flights or airports those flights came from; he doesn't even remember the exact time of the flight (he knows when he learned about it in school, but how long was that from the first attack?)
Could the NSA/FBI have done anything to stop the attack in this little a time frame, and would they from the unreliable warning of a stranger on the phone who's voice just broke? I assume with such a short time line they couldn't stop the terrorist until 9/11, but they could at least evacuate the buildings, maybe instigate some specific screening for middle-east citizens with pilot training bordering a flight in MD/NY that morning and search anyone that domes up for razor blades?
As an add on, if our time traveler went back further, to give a few months warning, would they be any more capable of stopping the attack before it happened?
finally, what are the odds our protagonist could do this without getting massive media attention, either by avoiding being tracked by the FBI or simply because the FBI respect his strongly stated wish to stay anonymous?