A pair of Department of Homeland Security officials told the Senate Wednesday that the government does not search for most of the people who overstay their temporary visas, a day after DHS said that nearly 500,000 people were still in the U.S. after having overstayed their visas last year.

"I didn't mean to imply that we're actually out monitoring them," Craig Healy, an assistant director at U.S. Immigration and Customs Enforcement, told Sen. Jeff Sessions, R-Ala., during Senate Judiciary subcommittee panel. Healy said that they review the universe of people who overstayed their visas and "prioritize" the deportation of people who went on to commit other crimes.

Their exchange came at the outset of a hearing on the federal government's failure to implement a biometric system to track entries into and exits from the country, as required by a 2004 law. A Customs and Border Patrol official said the program couldn't be implemented without causing "gridlock" at U.S. airports, a response that failed to allay bipartisan concern that the lack of this system is an ongoing national security threat.

"The biometric exit system is still not off the ground and that is unfortunate, very unfortunate, because it is a matter of national security," New York Senator Chuck Schumer, the Democratic leader-in-waiting, said during the hearing.

John Wagner, deputy assistant commissioner of field operations for U.S. Customs and Border Patrol, said the program couldn't be implemented without causing two-hour delays when boarding airplanes. "It's the placement of the technology and how you collect it to ensure that the person actually departed the United States," Wagner said. "There's no zone to do that."

These answers frustrated Democratic lawmakers who otherwise disagree with Sessions and other immigration hawks the issue of border security and deportations. "It's hard for me to envision that we can't figure out where to get a space to do this at an airport or seaport," said Senator Al Franken, D-Minn. "If you can't solve it in 11 or 12 years, how can we know it will ever be solved?"

DHS's report saying hundreds of thousands of people remained in the United States after having overstayed their visas drew complaints from both parties, but Sessions in particular.

"That is a population of individuals that is larger than any city in Iowa, New Hampshire, or South Carolina," Sessions said. Healy replied that about 3,000 of the people who had overstayed their visas were under investigation, a statistic Sessions cited to argue that President Obama's team has made no effort to implement the system or to deport people who overstay their visas, as long as those people "keep their nose clean" and do nothing to draw the attention of law enforcement or counterterrorism officials.

He said the lack of a biometric exit system was part of a broader failure by the Obama administration to implement federal immigration law.

"Our executive branch is on strike against the will of the American people," Sessions said. "Simply put, there is no border at all if we don't enforce our visa rules."