I think you need to take the same approach as the British during WW2 with the Enigma. They could decrypt the messages and know when attacks would happen, but if they stopped every attack, the Nazis would know and change encryption device. So you need to accept that some people will die and only mitigate the disaster in small but impactful ways.
If you went back and warned everyone about 9/11 the day before, you’d be locked up in gitmo for decades.
Anonymous bomb threat
You’d probably still be locked up. Too many questions would come you couldn’t answer without revealing you were a time traveller.
There is no answering! Make the call… Leave
Thinking the feds wouldn’t followup on a bomb threat especially when airplanes hit afterwards is wild.
2001 you could use a pay phone to stay anonymous.
You could walk in there and pull the fire alarm
I think you need to take the same approach as the British during WW2 with the Enigma. They could decrypt the messages and know when attacks would happen, but if they stopped every attack, the Nazis would know and change encryption device. So you need to accept that some people will die and only mitigate the disaster in small but impactful ways.
Like taking out seth macfarlane on a drinking bender on September 10th?