The US government has mistakenly granted citizenship to at least 858 immigrants who had pending deportation orders from countries of concern to national security or with high rates of immigration fraud.
The Homeland Security Department's inspector general found the immigrants used different names or birthdates to apply for citizenship with US Citizenship and Immigration Services.
They weren't caught because their fingerprints were missing from government databases.
The immigrants are not identified by name but Inspector General John Roth's auditors said they were all from "special interest countries" - those that present a national security concern for the United States - or neighbouring countries with high rates of immigration fraud.
In an emailed statement, the Department of Homeland Security said the findings reflect what has long been a problem for immigration officials - old paper-based records containing fingerprint information that can't be searched electronically.
DHS says immigration officials are in the process of uploading these files and that officials will review ``every file'' identified as a case of possible fraud.
DHS officials identified an additional 953 people who had been naturalised despite outstanding deportation orders, though auditors couldn't determine if those immigrants had digital fingerprints on file or not.
Roth's report said fingerprints are missing from federal databases for as many as 315,000 immigrants with final deportation orders or who are fugitive criminals. Immigration and Customs Enforcement has not reviewed about 148,000 of those immigrants' files to add fingerprints to the digital record.
ICE, the DHS agency responsible for finding and deporting immigrants living in the country illegally, didn't consistently add digital fingerprint records of immigrants whom agents encountered until 2010.
