Source, the Smithsonian.  I do not know if you could classify it a conspiracy.  More of a cover-up.  
According to the documentation at the Smithsonian, the government feared the public repercussions of the Sabre breaking the sound barrier, as the public was told the money needed for the X-1 project was all about the race to break the sound barrier.
Go to the Smithsonian, read all about it.  A subtext of the docs was printed in Air & Space as well.  How can you guys be plane fanatics and not get Air & Space?