One reason has been the design of airports in the United States. In most countries, international passengers depart from a separate terminal where customs officials collect biometric information of those exiting the country. But in the United States, passengers for domestic and international flights are often in the same terminals, making it difficult for officials to collect information on people who leave the country — a flight to Jackson, Miss., can be located next to one going to Japan.

Customs and Border Protection has been testing a number of biometric programs in partnership with several airlines in Atlanta, Boston, New York and Washington, funded by up to $1 billion collected from certain visa fee surcharges over the next 10 years.

John Wagner, deputy executive assistant commissioner at the Customs and Border Protection’s Office of Field Operations, said the program would look different at different airports. In some cases the airlines themselves would run the program, incorporating the biometric system into their boarding process. At some airports, Mr. Wagner said, the exit system would be staffed by Customs and Border Protection officers, similar to the situation passengers faced at the Emirates airline boarding gate at Dulles airport.

“We’re trying to figure out the most effective way to do biometric exit,” he said.

For example, in New York and Atlanta, the agency has partnered with Delta Air Lines to test a facial recognition system at the boarding gate for international flights.

The Delta system checks whether a passenger is supposed to be on the plane by comparing the individual’s face with a gallery of photos that the airline has of people on its travel manifest. It also checks the passengers’ citizenship or immigration status against various Homeland Security and intelligence databases. For American citizens, the facial scans are checked against photos from State Department databases.

In Boston, Customs and Border Protection, in cooperation with JetBlue airlines, is testing a facial recognition system for travelers flying to Aruba.

Mr. Wagner said the agency hoped to begin using the biometric exit system at all airports with international flights in 2018.