To plan the best attacks they can.
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Terrorists usually do. They often have separate teams. One team is the surveillance team. They take pictures, video, analyze the security of their target, ask people who work or live near their target all about it. In short they find out everything about it in order to plan the attack. They do dry runs, they often analyze the aftermath of their attacks. The surveillance phase/planning phase usually takes around 2 years or more. There are roughly 16 steps to this. I don't have space to list them all. Another team is the one that actually carries out the attack. They will often train physically, with weapons, and train in military tactics required for the attack. Another team is the command element. They are the leaders who pick the targets, the dates, times, who will carry out the attack and where. Another team is made up of specialists. They specialize in making bombs, or gear or aquiring it in order to carry out the attack. Terrorists (domestic or Islamic extremists) attacks are usually not spontaneous.
Examples of planned attacks that featured some or all of these elements: 9-11, the attacks that happened in France recently, the San Bernardino shootings, and most terrorists acts that have occurred in Israel for the past 20 years.
Domestic Terrorists (non ISIS ect) mass shooters often plot and train to a lesser degree. The man responsible for the Sandy Hook Masacre did a lot of target pratice, Holms who was responsible for the Aurora, Colorado mass shooting studied how to and made bombs which he left in his apartment for police. The 2 boys responsible for the Colombine Massacre also did a lot of target pratice and killing.
Murders (street crime, organized crime, and serial killers) Often do a lot of planning in their attacks and training with firearms but to a much lesser extent than domestic terrorists, and Islamic Radicals. Some don't do any planning or training. They do spontaneous murders.
I know these things because I have recieved counter surveillance training and work in the Criminal Justice field.
The in's and outs of murders, mass shooters, and terrorists as to how much training in/with firearms/ military style training and planning they do is very broad as is the degree to which they do these things.0