Nearly 20 years ago, Congress passed a law requiring the federal government to develop a system to track people who overstayed their visas. After the attacks of Sept. 11, 2001, an entry and exit tracking system was seen as a vital national security and counterterrorism tool, and the 9/11 Commission recommended that the Department of Homeland Security complete a system “as soon as possible.” Two of the 9/11 hijackers, Satam al-Suqami and Nawaf al-Hazmi, had overstayed their visas.

Since then, the federal government has spent millions of dollars on the effort, yet officials can only roughly estimate the number of people in the United States illegally after overstaying visas.

Officials blame a lack of technology to conduct more advanced collection of data like iris scans, resistance from the airline and tourism industry because of cost, and questions about the usefulness of tracking people exiting the country as a counterterrorism measure.

Some experts also note that a sizable number of those who overstayed their visas are highly skilled workers who come under the H-1B program or are foreign students.