The Obama administration is cracking down on immigrants in the U.S. who have overstayed the terms of their visas by using a system that automatically checks multiple national security, immigration and law enforcement databases at the same time, a senior Homeland Security Department official said.

The common practice has been to make manual checks of individual databases. The new system has already identified dozens of investigative leads, said John Cohen, deputy counterterrorism coordinator at the Homeland Security Department.

The immediate focus is on identifying people who have overstayed their visas and who pose a potential threat to national security or public safety, Cohen said.

Some of the 19 Sept. 11 hijackers were in the U.S. in violation of their visas, in some cases because they did not attend a school they said they would on their application for a student visa, or their visas had expired.

The 9/11 Commission saw the visa system as a major vulnerability and recommended completing a biometric system that would log immigrants out as they left the U.S. This program, however, was never fully implemented. Homeland Security Secretary Janet Napolitano has said the exit system called for by the commission is expensive, and the government has put other policies in place since 2001 to address the same issue for a lot less money.

Automating these checks is the latest of those policies. Until now, if an investigator wanted to vet a visa applicant, it would require manual checks of many databases. This policy left room for mistakes, such as someone entering the wrong spelling of an immigrant's name, which might not turn up critical national security information.

It took years for the government to effectively connect the various terror watch lists held by different government agencies after 9/11 exposed major holes in national security caused by keeping all of these lists separate. Now that those systems are interconnected and have greater search capabilities, DHS is taking it a step further to do immigration checks which will automatically scan all variants of immigration status — be it refugee or asylum — while at the same time check other criminal and national security databases and run searches on variations of names.

"The concept of interconnecting these systems isn't really an earth-shattering idea," Cohen said. But the systems that immigration data will be run against have finally reached a point where they can handle a broad check, he said.

The department is also regularly checking the systems for people whose visas haven't expired — in some cases, as often as daily or weekly, Cohen said.

Such a review process addresses the reality that information about a person's potential terrorism ties might not be clear to the intelligence community until after a visa has been issued. It would have probably flagged the Nigerian man believed to be behind the attempt on Christmas Day 2009 to blow up an airliner over Detroit, Cohen said.

Until recently, there was a backlog of 1.6 million people who had overstayed their visas in the U.S. Cohen said the department put this information through the automated checks and determined 800,000 of those people had either changed their visa status or had already left the U.S.

Of the remaining 839,000, Cohen said the department vetted everyone for potential national security and public safety concerns. With the help of the National Counterterrorism Center, the department determined there were about 2,000 people in the U.S. who warranted investigation. In some cases, those people were already under investigation or had left the country, leaving several dozen leads for ICE agents to pursue.