According to this document, 29 messages from outer-space were intercepted and subsequently published in a National Security Agency technical journal. This document is a "key" created in order to interpret the messages. So we don't really know what the messages actually said, or if they were even successfully unlocked, but this document appears to prove that the NSA has in fact intercepted at least 29 radio messages of extra-terrestrial origin. When these messages may have been picked up, how they were heard, is anyone's guess at this point. But this seems to prove the we have indeed heard from aliens and the NSA knows it.
The pdf document is linked here directly from the NSA, so there is no question that the document is authentic:
http://www.nsa.gov/public_info/_files/ufo/key_to_et_messages.pdf
Some additional information is available at this link:
http://www.ufodigest.com/article/official-et-disclosure-nsa-document-admits-et-contact-kevin-w-smith
