Who's to know? I suspect at ATO a good portion of the landscape crew. I have my reasons for suspecting this but no clear facts.
possibly janitorial, landscape, food services...like everywhere services are outsourced. You would expect something in outsource contract to require verification of right to employment. Its outsourced to JLL oursourcing now days. Probably 20% or more of normal Amgen staff are Permanent Residents with Green card. HR is pretty thorough with normal staff hiring checks.