There have been a lot of questions like this asked by the general public and by the media. I am sure that they are asked with genuine ignorance, but just a little common sense would tell you that fighter jets would be ineffective.
Many attempts would be made to contact the aircraft concerned by ATC and it would not be possible to know their final destination until it was too late.
If you saw the video of the first aircraft hitting the Trade Center you see the fireman look up at the low flying aircraft then return to his job. The camera pans left and it was only a couple of seconds before impact that the realisation dawned of what was happening.
It has also been said that the White House has anti aircraft missile defences on it's roof. Even if an aircraft the size of a 767 was hit several times it would continue on its crash course.