From HSToday:

An alarming increase in recent Islamic State-inspired terrorist plots has contributed to rising concerns among counterterrorism officials about Americans and other Westerners traveling to Iraq and Syria to obtain terrorist training and then return home to carry out attacks. Beginning in the early 1990s, law enforcement authorities became increasingly concerned about “lone wolf” terrorism, which the University of Marlyand-based National Consortium for the Study of Terrorism and Responses to Terrorism (START) defines as “persons who had no help in planning, preparing, or committing the terrorism incident.”