https://www.grassley.senate.gov/news/news-releases/grassley-seeks-information-noncitizens-working-without-authorization
“The use of non-work SSNs to obtain employment and work illegally is not an insignificant problem. The OIG report stated that approximately 364,000 people with non-work SSNs reported billions of dollars in wages over a 3-year tax period from 2014-2016. It is the responsibility of DHS to investigate aliens who are in violation of the laws of the United States, including those working without authorization.”
Grassley digging