Why weren't military personnel in air shooting down planes after first 2 attacks on 9/11/01?

I have been asking myself this question for the past 9 years. I know at one point that morning there was discussion of military jets prepared to take down hijacked airplanes not responding to air traffic control, however I don't understand why the one that Cleveland center identified as having a bomb on board as well as it turning around and heading back toward the capitol was not shot down and also how was a commercial flight allowed so close to all of our military and government operations in Washington? I am not interested in conspiracy theories as an answer thank you. There's a difference between knowing what happened and thinking you know what happened.