Visa applicants lining up at the U.S. Embassy in New Dehli, India. (AP photo)

(CNSNews.com) -- A total of 527,127 foreign nationals who were admitted to the United States on non-immigrant visas and who were supposed to leave the country in 2015 failed to do so, Department of Homeland Security (DHS) officials told a congressional subcommittee on Tuesday.

"In FY 2015, of...nearly 45 million nonimmigrant visitors, DHS calculated a total overstay rate of 1.17 percent, or 527,127 individuals,” DHS officials said in a joint statement submitted to the House Subcommittee on Border and Maritime Security, whcih held a hearing on "Overstaying Their Welcome: National Risks Posed by Visa Overstays."

Some visitors stayed beyond their authorized visit, but left the country by Jan. 4, 2016, reducing the suspected overstays to 416,500, the statement continued.

In January, DHS released its first Entry/Exit Overstay Report, which contains data on visa overstays, defined as "a nonimmigrant who was lawfully admitted to the United States for an authorized period but stayed in the United States beyond his or her lawful admission period.”

U.S. Customs and Immigration Enforcement (ICE) told Sen. Jeff Sessions (R-AL), chairman of the Senate Subcommittee on Immigration and the National Interest, that it deported 2,456 individuals who had overstayed their visas in 2015 - the lowest number since President Obama took office, according to a May 6 press release.

However, DHS noted that the total number of visa overstays represent only visitors traveling through air and sea points of entry for purposes of business or pleasure, and do not include foreign nationals who crossed the U.S. border by land.

The African nation of Djibouti had the highest percentage of visa overstays (27.67%), according to DHS, but overstay rates for several other countries were well over 10 percent, including: Bhutan (24.89%), Eritrea (19.28%), Laos (18.44%), Burkina Faso (18.01%), Chad (17.43%), Federated States of Micronesia (16.00%), Mauritania (13.49%), Georgia (12.44%), Gambia (11.20%), Liberia (11.93%), and Afghanistan (10.86%).

“While Congress has in recent years paid a great deal of attention to securing our southern border (and rightfully so), less attention has been focused on successfully addressing visa overstays,” said Rep. Filemon Vela (D-TX), the subcommittee’s ranking member.

“The approximately 527,000 individuals who overstayed in fiscal year 2015 is a far greater number than the 331,000 individuals who were apprehended along the U.S.-Mexico border that year, illustrating the scope of the overstay problem,” Vela said.

Subcommittee chairwoman Martha McSally (R-AZ), estimated that nearly 60 percent of individuals illegally in the United States are visa overstayers. She emphasized the risk of terrorists being among that group.

“Time and time again, terrorists have exploited the visa system by legally entering America. The 9/11 Commission put it this way: ‘For terrorists, travel documents are as important as weapons’,” McSally said.

“I am concerned that there are unidentified national security and public safety risks in a population that large, which has historically been the primary means for terrorist entry into the United States,” she added, noting that “four of the 9/11 hijackers, as well as the man who attempted to bomb the U.S. Capitol in 2012, had overstayed their visas.”

In deporting visa overstayers, those considered high risk are dealt with first, Craig Healy, the assistant director for national security investigations at ICE, told the subcommittee.

“ICE prioritizes nonimmigrant overstay leads through risk-based analysis. A targeting framework consisting of ten tiers was developed in close consultation with the intelligence and law enforcement communities to ensure that our national security and public safety concerns are prioritized,” he said.

Healy testified that approximately 10,000 visa overstay leads which ICE handled in 2015 were considered high risk, although 4,000 of those were later determined to be in compliance. Of those considered high risk, ICE made 1,900 arrests.

But Rep. Lamar Smith (R-TX) said that all visa overstayers, not just high risk ones, need to be deported.

“My concern with the administration’s actions is that by sending so few home, by deporting such a small percentage of the visa overstayers, the message they are sending wide and far is ‘just get into the country, if you are not convicted of a serious crime you’re going to be allowed to stay, you’re going to pass ‘go’, you’re going to get the money’ and that is the wrong message to send,” Smith said.

However, Rep. Sheila Jackson Lee (D-TX) said that dealing with high risk visa overstays should be the federal government’s main concern. “I believe that the President’s approach of focusing on those who would do us harm is the most important approach,” she said.

Pointing out that the visa overstay rate is only 1.17 percent, “today, I’m going to say that the Congress has some higher responsibilities right now,” Jackson Lee said. Gun control should be the top priority after the Orlando shooting last Sunday, she added, noting that the shooter, Omar Mateen, was a U.S. citizen.

Ascertaining that visitors with expired visas have actually left the country is hampered by the fact that there is currently no system to collect exit biometric data, such as photographs and fingerprints, which can be matched to the biometric data that is already collected when they arrive, said John Wagner, deputy executive assistant commissioner in the Office of Field Operations at U.S. Customs and Border Protection (CBP).

Wagner said that the CBP has been testing an automated biometric exit verification system and plans to begin implementing such systems at airports and seaports in 2018.

However, he noted that collecting biometric data at land border crossings would be far more complicated.

"Putting a biometric exit system in place is, as the 9/11 Commission noted, ‘an essential investment in our national security,’ because without a viable biometric exit system, visa holders can overstay their visa and disappear into the United States, just as four of the 9/11 hijackers were able to do,” McSally said.