Let's see... neighbors and people who knew Syed Farook will be surprised that he was involved. He's such a quiet, nice Muslim. He blended in so well we would have never guessed he was capable of doing something like this.
That's the problem. They blend in so well, act normal and live everyday regular lives for years and years. These terrorists are deep, deep undercover posing as average American (or French, Belgian, etc.) citizens. Coworkers and neighbors know them as regular people and it comes as a shock when they turn out to be terrorists. There are about 6.5 million Muslims living in America. A vast majority of them will never do anything like this. But hundreds are just secretly, gleefully watching the news of Muslims killing westerners... and they're just waiting for the right time.