U.S. Citizenship and Immigration Services (USCIS) says election officials have used the tool to check the information of more than 33 million voters — a striking portion of the American public, considering little information has been made public about the tool's accuracy or data security.

The latest update to the system, known as SAVE, took effect Aug. 15 and allows election officials to use just the last four digits of voters' Social Security numbers — along with names and dates of birth — to check if the voters are U.S. citizens, or if they have died.

The upgrade makes the tool far more accessible, since it now aligns with the information most states collect or have access to for most voters. But the Department of Homeland Security (DHS), which houses USCIS, has not responded to questions about the system from members of Congress, and numerous election officials NPR spoke with expressed concern about what else the Trump administration could do with the data it acquires from states.