War of the Worlds
The United States government would not be without reason to supress news of extraterrestrial contact. In 1928 Orson Welles' War of the Worlds was broadcast on radio in the Eastern United States. When the broadcast started it was stated that the story was fictional; many listeners missed this though because they changed to the station after first seeing what another station had. The radio drama told the story of an alien invasion of New Jersey. Panic quickly spread and thousands paniced, flooding the local authorities with calls. This was the type of reaction that happened when the alien event was fictional.
If true contact occured between Earth and an extraterrestrial race the situation would be much worse. Panic would spread across America only it would be real contact and not just a radio program. To avoid this the government would not stop from concealing the existence of extraterrestrial bilogical enities thinking that the country and the world would not be able to handle such news.