When a  mass murder is committed by anyone the deaths are the horrific results.  Trying to analysis a specific group as the one most likely involved requires full and complete information not simply pick one set of culprits. There have been 844 death in mass shooting in the US in the last 50 years https://www.washingtonpost.com/graphics/national/mass-shootings-in-america/    While the overall number of shooters is 132 there are several common factors of who apply to more than one shooter.  Muslims were involved involved with no more incidents that wackos killing abortion providers.  The need to prevent these killings is obvious.  Focusing on one group while sidestepping other common factors has proven to be fruitless as the shooting by non-Muslims far exceeds those committed by Muslims.  I want the system that looks at all the information and takes more than one factor into consideration.  Vetting Muslim refugees is but one step.  Lets not look at others potential common factors.