SHA-2 is an open algorithm and it uses as its constants the sequential prime cube roots as a form of "nothing up my sleeve numbers".  For someone to find a weakness or backdoor in SHA would be the equivalent of the nobel prize in cryptography.   Everyone who is anyone in the cryptography community has looked at SHA-2.  Not just everyone with a higher degree in mathematics, computer science, or cryptography in the last 20 years but foreign intelligence agencies and major financial institutions.    Nobody has found a flaw, not even an theoretical one (a faster than brute force solution which requires so much energy/time as to be have no real world value).

To believe the the NSA has broken SHA-2 would be to believe that the NSA found something the entire rest of the world combined hasn't found for twenty years.  Also NIST still considers SHA-2 secure and prohibits the use of any other hashing algorithm (to include SHA-3 so far) in classified networks.  So that would mean the NSA is keeping a flaw/exploit from NIST compromising US national security. 

Anything is possible but occam's razor and all that.