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, according to an internal Homeland Security audit released on Monday.

The Homeland Security Department’s inspector general found that the immigrants used different names or birthdates to apply for citizenship with US Citizenship and Immigration Services and such discrepancies weren’t caught because their fingerprints were missing from government databases.

The report does not identify any of the immigrants 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 neighboring countries with high rates of immigration fraud.

The report did not identify those countries.

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.

The gap was created because older, paper records were never added to fingerprint databases created by both the now-defunct Immigration and Naturalization Service and the FBI in the 1990s. 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.

The government has known about the information gap and its impact on naturalization decisions since at least 2008 when a Customs and Border Protection official identified 206 immigrants who used a different name or other biographical information to gain citizenship or other immigration benefits, though few cases have been investigated.

Roth’s report said federal prosecutors have accepted two criminal cases that led to the immigrants being stripped of their citizenship. But prosecutors declined another 26 cases.

ICE is investigating 32 other cases after closing 90 investigations.

ICE officials told auditors that the agency hadn’t pursued many of these cases in the past because federal prosecutors "generally did not accept immigration benefits fraud cases."