Do first-person shooters influence real-life violence?

I'm doing some research for a media study; I want to know what different people think about this subject. Do first person shooter games, like Call of Duty and Battlefield, where the player takes on the role of a gunman, have an impact/influence on real life violence? Here's a few examples of where it could be argued that these games do in-fact encourage violence:

Anders Breivik- Perpetrator of the Norway attacks 2011 in which a total of 77 lives were lost. Anders claimed that he 'trained on Call of Duty' before carrying out these attacks

Aaron Alexis- Shot and killed 12 people earlier this year, in September at the Washington Navy Yard. He would play games like Call of Duty and similar shooting games for up to almost 18 hours a time. It was claimed that he heard voices in his head after playing these games. Friends and family believe this pushed him to to do what he did.

I would like your input on this subject; what do you think about it, do you agree? Do you disagree? Why? Please just be as open as possible. When leaving a comment please leave your gender and age group. Thank you :)