Screening ground employees is an impractical solution, as once past security, there is access to any number of potentially dangerous tools, etc. required to do one's job in the heavily mechanized ramp environment. What may be improved is the screening of these employees for criminal history. Several of the ground service companies to whom ground services are outsourced have employees who exalt in wearing their gang symbols and colors on the ramp, something I have never seen tolerated with airline employees.