Almost anyone can purchase ordinary commercially available materials, fashion them into a crude but nonetheless lethal bomb. All that is needed is the willingness and the motivation to act. Fanatical and mentally disordered individuals who are not trained terrorists are the most unpredictable terrorists. Without any strategy, they may contemplate killing a lot of people simply because, for example, they are not their co-religionists.
Amateur terrorists are even more dangerous than professionals. Amateurs have not had time invested in their training and are expendable. Professional terrorists are part of an organization with a command and control structure, follow orders from some external force and "have an address" that can be retaliated against. An established organization has a modus operandi that enables investigators to build a profile and to anticipate events.