Sudden jihad syndrome was coined by Daniel Pipes, an excellent scholar. Jihad, such as the recent attack in Chattanooga seem to done by “nice” people. How do we explain “nice Muslims” who commit jihad? Some nice Muslims are simply do not follow much of Islam. They are Islam, light. Then there are other nice Muslims who know all of Islam, including jihad, who are pleasant because they need to live in our society. But deep within, they have the heart of a jihadist which can manifest suddenly.
How can we tell the difference? Are they really a friend or a deceiver? There is no way to know the heart of a Muslim.