More than 800 illegal immigrants from countries of concern who were set for deportation were mistakenly granted U.S. citizenship because the Department of Homeland Security didn’t have their fingerprints on file, according to an internal audit released Monday.

The Homeland Security Department’s inspector general found the immigrants used different names or birthdates to apply for citizenship with the U.S. Citizenship and Immigration. In the case of 858 immigrants from "special interest countries or neighboring countries with high rates of immigration fraud," the discrepancies weren’t caught because their fingerprints were missing from government databases.

A few even managed to get aviation or transportation worker credentials, though they were later revoked. One became a law enforcement officer.

The findings were released, incidentally, as authorities were investigating a string of weekend attacks, allegedly connected to foreign-born suspects.

The inspector general report could further fuel warnings about immigration security. The report warned that when immigrants become naturalized, "these individuals retain many of the rights and privileges of U.S. citizenship, including serving in law enforcement, obtaining a security clearance, and sponsoring other aliens’ entry into the United States."

The tally in the report was provided by the administration in mid-2014.

But the problem could be even worse. According to the audit, as of November 2015, the administration has found 953 more "who had final deportation orders under another identity and had been naturalized," some of whom were from countries of concern.

DHS Inspector General John Roth also found fingerprints 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.

“This situation created opportunities for individuals to gain the rights and privileges of U.S. citizenship through fraud,” Roth said. “To prevent fraud and ensure thorough review of naturalization applications, USCIS needs access to these fingerprint records.”

Roth added that DHS has agreed to the recommendations made in the audit and that ICE has plans to “review the eligibility of each naturalized citizen whose fingerprint records reveal a deportation order under a different identity.”

The gap in fingerprints 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 the agency hadn't pursued many of these cases in the past because federal prosecutors "generally did not accept immigration benefits fraud cases." ICE said the Justice Department has now agreed to focus on cases involving people who have acquired security clearances, jobs of public trust or other security credentials.

Mistakenly awarding citizenship to someone ordered deported can have serious consequences because U.S. citizens can typically apply for and receive security clearances or take security-sensitive jobs.

At least three of the immigrants-turned-citizens were able to acquire aviation or transportation worker credentials, granting them access to secure areas in airports or maritime facilities and vessels. Their credentials were revoked after they were identified as having been granted citizenship improperly, Roth said in his report.

A fourth person is now a law enforcement officer.

Roth recommended that all of the outstanding cases be reviewed and fingerprints in those cases be added to the government's database and that immigration enforcement officials create a system to evaluate each of the cases of immigrants who were improperly granted citizenship. DHS officials agreed with the recommendations and said the agency is working to implement the changes.

The Associated Press contributed to this report.