How did America know it was terrorists who hijacked on 9/11?

did anyone not pay attention that day and the days after. i am sure that that whom ever said that it was terrorists had alot more information than you do. the people on flight united 93 gave descriptions of the people that were hijacking the plane. that is were thay got the information on who was doing those terrible things. go online find united 93 the movie buy it and watch it. also doe searches and read all the material that you can find about 9/11 and you may learn something.