The discovery of non-citizens on voter rolls and voting seems to center around DMV data.
For example, the ERIC (Electronic Registration Information Center) program and SAVE (Systematic Alien Verification for Entitlements), both link DMV data to voter registration.
Currently, 19 states and the District of Columbia have passed laws allowing undocumented immigrants to obtain standard driver's licenses. These states include California, New York, New Jersey, Minnesota, and Colorado.
Proponents argue better road safety.
The problem is the system is being gamed to get non-citizens onto voting rolls through the process of getting them a drivers license.
The "safer streets" push masks the true purpose of these programs - illegal votes.
Note that some states are refusing to turn DMV data over to ICE. They know this data reveals non-citizens with driver's licenses.