My guess is the feds got some sort of cryptic vague warnings from a foreign intelligence service prior to the attack (Saudis). A foreign intelligence service that wants to save face on both sides by giving a warning that makes them look like they are being helpful, yet not specific enough to stop the attack. Saves face with their relationship with the US government and then also with their constituency at home that supports these kinds of attacks and will figure out how to profit from them.