Here is an investigative piece by Paul Sperry, for the New York Post, that describes the wider connections amongst home-grown American terrorists: http://nypost.com/2016/06/18/why-the-lone-wolf-terrorist-is-a-myth/
For a more in-depth study of the problem, Stephen Coughlin's book is a good place to start: https://www.amazon.com/Catastrophic-Failure-Blindfolding-America-Jihad/dp/1511617500
