33 million voters have been run through a Trump administration citizenship check

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Tens of millions of voters have had their citizenship status and other information checked using a revamped tool offered by the Trump administration, even as many states — led by both Democrats and Republicans — are refusing or hesitating to use it because of outstanding questions about the system.

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.

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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.

"There's still uncertainty about what is happening, what happens to the data that are shared with USCIS," said Charles Stewart, a political science professor who directs the MIT Election Data and Science Lab. "I don't know if this means that the USCIS now has a depository of one-sixth of all [the country's] registered voters."

In recent months, several Republican-led states have brokered new agreements with USCIS to use SAVE, or announced the results of SAVE reviews. Ohio election officials will begin removing from their rolls thousands of inactive voters that SAVE identified as deceased.