Sen. Ron Johnson, R-Wis., is demanding answers from U.S. Citizenship and Immigration Services after a watchdog report found the agency granted citizenship to at least 858 people who were ordered to be deported or removed.

The Department of Homeland Security Inspector's General report found those people were mistakenly granted American citizenship from countries of concern to U.S. national security, or neighboring countries with high rates of immigration.

Johnson, who chairs the Homeland Security and Governmental Affairs Committee, said that report is a huge failure that demands further study.

"This failure represents a significant risk to America's national security as these naturalized individuals have access to serve in positions of public trust and the ability to obtain security clearances and sponsor other aliens' entry into the United States," Johnson wrote in a letter to DHS Secretary Jeh Johnson on Monday.

Johnson added that without changes, "our national security may be jeopardized by an inability to adequately assess whether foreign nationals pose a security risk."

According to the IG report, the people were able to get citizenship by using different names or birth dates in applications. The discrepancies weren't caught because their fingerprints were missing from federal databases.

The report also found that ICE has failed to review roughly 148,000 fingerprint records into databases, which means it becomes easier for immigrants to be granted citizenship without officials fully knowing someone's immigration and criminal histories.

Johnson demanded that DHS disclose the number of civil and criminal cases opened against any of the 858 immigrants, as well as details about the 90 ICE investigations into people caught closed by ICE and another 32 cases opened since March 2015.