Tracking the entry and exit of foreign travelers legally visiting the U.S. seems simple enough.
In reality, it is an insurmountable challenge.
The best tracking occurs at air and sea ports of entry, yet even those systems are not comprehensive.
The land borders are even trickier. Ports of entry along the southern border are focused primarily on screening incoming traffic, not checking who is leaving.
As a result, it is nearly impossible to know how many people remain in the country long after they were supposed to leave.
The U.S. Department of Homeland Security estimated 701,900 visa overstay incidents in fiscal 2017 – and that number accounts only for air and sea travel.
That's about double the number of migrants caught crossing into the U.S. from Mexico illegally during the same period, according to Border Patrol data.
"For the past 10 years, the primary mode of entry to the undocumented population has been to overstay temporary visas," said Robert Warren, a demographer who spent years tracking the unauthorized immigrant population for the U.S. government.
The visa overstay population has attracted attention in recent years but remains overshadowed by the Trump administration's focus on border walls.
According to Warren's research for the Center for Migration Studies of New York – an international migration and refugee protection think tank – visa overstays account for an estimated 44 percent of the unauthorized immigrant population in the United States.
Due to the large number of unauthorized immigrants, U.S. authorities have focused enforcement on those who appear to pose a threat to national security or rack up criminal records during their stay.
As for the rest? "There's no one looking for you," said Jessica Vaughan, director of policy studies at the Center for Immigration Studies, an institute that advocates for tighter immigration control.
The alarm on visa overstays first sounded in earnest in 1993, after the bombing of the World Trade Center in New York. Some of the perpetrators had entered the U.S. legally on student and tourist visas, then overstayed.
Similarly, two of the hijackers in the Sept. 11, 2001, attacks were in the U.S. on overstayed visas.
"It became generally realized that we had no idea which people overstayed their visas," said Vaughan, who previously worked as a Foreign Service officer for the State Department, which issues visas.
There were many calls for a robust entry-exit system to track overstays, but implementation has been slow and riddled with technology glitches and privacy concerns.
At airports, exits used to be tracked via a paper system, as travelers handed over perforated I-94 entry form tabs to their airlines upon departure. Now, U.S. authorities track exits electronically via airline manifests.
The US-VISIT program in 2004 added identity verification with digital fingerprinting upon entry to the U.S. – a step that cut down on passport and visa fraud – but a matching exit procedure was not widely implemented.
Now, the technology has advanced to face-recognition systems. At a kiosk, travelers scan their passports and then pose for a quick photo, and the machine determines if it is the same person.
At least 17 major airports – including Atlanta, New York's JFK and Miami – are using face recognition for exiting foreign travelers, according to U.S. Customs and Border Protection.
Equipping all U.S. air and sea ports eventually with similar exit tracking is feasible, experts say. The land border is an entirely different matter.
On any given day, CBP is processing nearly 700,000 travelers through land ports of entry – more than 254 million people annually. For the most part, they are either citizens of the U.S., Mexico or Canada, or legal permanent residents, who travel back and forth across the border for commercial trade, tourism, work, school, family visits or a simple trip to the store.
On the border with Mexico, CBP will sometimes monitor and stop southbound traffic, but it is mostly to look for drug money or guns being smuggled to Mexico.
Past efforts to implement a universal biometric exit process have always reached the same conclusion: it can't be done without major disruptions to cross-border life as we know it.
The U.S. Government Accountability Office noted in a 2007 report that such a system would be costly, require new infrastructure and produce major traffic congestion and wait times as each departing vehicle is stopped and all occupants processed.
But the government hasn't given up.
Last year, CBP piloted the use of face recognition technology at Arizona's Nogales and San Luis ports of entry for arriving travelers, with the program expected to spread to other land crossings. Tests on an exit procedure are expected.
At Texas' Anzalduas Port of Entry, CBP also tested face recognition on people inside vehicles moving under 20 mph last year.
There was also a 2016 experiment at the Otay Mesa border crossing using fingerprint devices on third-country nationals leaving the U.S.
Meanwhile, an entry-exit system is being developed at the U.S.-Canadian border – but not based on biometrics.
The two countries now share biographical data on northern border crossings of third-country nationals, as well as U.S. citizens, with information sharing on Canadian citizens expected in the near future.
That kind of information sharing is still being explored with Mexico.
Last fiscal year at the San Ysidro-El Chaparral crossing, CBP partnered with Mexico's National Migration Institute in a pilot that exchanged biographic border crossing data on Mexican citizens crossing on foot, using radio frequency identification, or RFID, which are tags built into modern travel documents, to mark both entries and exits.
But RFID technology has been iffy in the past.
During an RFID pilot project to track exits, DHS found RFID readers did not detect a majority of travelers' tags, the GAO reported in 2007. Plus, the tags are not physically tied to a traveler, meaning a document may leave the country but the person to whom it was issued may remain in the U.S.
The lack of complete data on who is staying and who is leaving has made it challenging to understand the bigger picture, as well as to track down specific offenders.
However, some notable findings have emerged.
According to DHS' most recent data, an estimated 1.3 percent of the nearly 52.7 million legal admissions into the U.S. overstayed in fiscal 2017 – not counting the land borders.
By the end of that fiscal year, DHS suspected that about 607,000 people had overstayed visas. But it is a moving target; by May 1, 2018, that number had fallen to 421,000 as some overstays returned home.
The highest rate of overstays in fiscal 2017 occurred on student and exchange visitor visas, with 40 percent from four countries: China, Saudi Arabia, India and South Korea. The data exclude Mexican and Canadian student and exchange visitors.
The category with the largest number of overstays came from short-term visitors, with about 20 percent from two countries –– Brazil and Venezuela. That category does not include countries participating in the Visa Waiver Program, which allows their citizens and Americans to travel without visas for short-term business and tourist stays.
However, Visa Waiver nations still accounted for 19 percent of total overstays, led by the United Kingdom and France.
Air and sea travel by Mexicans and Canadians were factored separately, with an estimated 101,000 overstay incidents from Canada and nearly 53,000 from Mexico.
However, the overall accuracy of the DHS data, culled from air and sea carrier passenger manifests, has been questioned.
When the agency reported that in fiscal 2016 it had not been able to verify the departures of 628,799 foreign visitors, separate research by the Center for Migration Studies found nearly half had likely left the U.S. but their departures went unnoticed.
That puts the estimated visa overstays at closer to 300,000, the study's author, Warren, concluded.
Warren has been using a different method to track the unauthorized immigrant population – including visa overstays – using the Census Bureau's American Communities Survey, which encompasses land crossings as well.
His calculations identify Mexico as the leading overstay country, estimating some 50,000 Mexicans arrived in the U.S. in 2016 and overstayed their visas, followed by citizens from India, China and Venezuela.
Suspected visa overstays are run through national security and law enforcement databases to look for public safety threats. That's where the enforcement is primarily focused.
Last year, U.S. Homeland Security Investigations created a specialized unit, the Counterterrorism and Criminal Exploitation Unit, to investigate such cases.
Lower risk overstays are filtered down to immigration officers around the country, although it's unclear how many leads generate arrests.
Investigating these cases is more complicated than it ought to be, according to the Office of the Inspector General.
The federal oversight agency reported in 2017 that investigators had to piece together information from dozens of systems and databases that were not integrated, some "stove piped" so as to not share information. Password problems and confusion over which systems needed to be checked were also an issue, as well as access to real-time information on a person's legal status to see if the person had corrected the overstay issue, the report found.
"An agent spends a lot of time figuring out if a person really did leave, maybe they left by a land port," Vaughan said. "If they didn't leave, where the heck are they?"
In February, Atlanta rapper 21 Savage was arrested by ICE during a targeted operation, causing the Grammy-nominated act to miss his performance at the awards ceremony. The British citizen is accused of overstaying his childhood visa. His lawyers said the rapper had applied to renew his legal status, but the application was pending at the time of his arrest.
Overstaying a visa is a civil immigration offense, not a crime. Penalties can include being barred from returning to the U.S. for several years.
Immigration experts say the government is better off focusing on overstays on the front end.
DHS is trying out a new program that alerts visitors from Visa Waiver countries by email that the end of their authorized admission date is near, to "improve traveler awareness," the agency said. The notification program is expected to expand to other types of visitors.
Experts also say the State Department should use the overstay demographics data to adjust applicant screening.