Philadelphia International Airport is the latest in a growing number of U.S. airports that use facial recognition as part of a biometric screening system for travelers.

The 45-day pilot program starting Tuesday, Jan. 21, will involve three international gates at PHL, where U.S. Customs and Border Protection agents process departing passengers.

Similar systems already are in use to some extent at Newark Liberty International and John F. Kennedy International airports, according to the CBP.

Biometric screening cross-checks facial scans with photos already on file with the federal government as a way of verifying travelers' identities.

At Philadelphia, three types of biometric systems — veriScan, NEC and SITA — will be used at gates A15, A16 and A17 for select outbound international flights on Qatar, British Airways, Lufthansa and American Airlines.

According to the CBP, U.S. citizens and "exempt aliens" are not required to have their pictures taken.

"Travelers who do not wish to participate in this facial comparison process may notify a CBP Officer or an airline, airport or cruise line representative in order to seek an alternative means of verifying their identities and documents," the agency said.

The CBP says it discards all photos of U.S. citizens within 12 hours of identity verification.

According to the CBP, biometric screening is in use for departing travelers at Newark Liberty International and John F. Kennedy airports as well as these airports:

- Atlanta (ATL)
- Austin (ABIA)
- Boston (BOS)
- Chicago (ORD)
- Dallas (DFW)
- Detroit (DTW)
- Fort Lauderdale (FLL)
- Houston Hobby (HOU)
- Houston (IAH)
- Las Vegas (LAS)
- Los Angeles (LAX)
- Miami (MIA)
- Minneapolis/St. Paul (MSP)
- New Jersey (EWR)
- New York (JFK)
- Orlando (MCO)
- Portland (PDX)
- San Antonio (SAT)
- Salt Lake City (SLC)
- San Diego (SAN)
- San Francisco (SFO)
- San Jose (SJC)
- Seattle (SEA)
- Tampa (TPA)
- Washington Dulles (IAD)
- Washington Reagan (DCA)

CBP said biometric screening is in place for arriving travelers at the following U.S. airports:

- Atlanta (ATL)
- Detroit (DTW)
- Dulles (IAD)
- Ft. Lauderdale (FLL)
- Houston (IAH)
- John F Kennedy (JFK)
- Las Vegas (LAS)
- Los Angeles/Tom Bradley (TBIT)
- Miami (MIA)
- Orlando (MCO)
- San Diego (SAN)
- San Jose (SJC)

Biometric screening is used at five U.S. ports for arriving cruise travelers, including Bayonne via Cape Liberty Cruise Terminal, according to federal security officials.

The extensive rollout of a “biometric entry-exit tracking system” for travelers was called for in a 2017 executive order by President Trump, the same one that included a controversial travel ban "to protect the Nation from terrorist activities by foreign nationals."

In the executive order, Trump said the biometric rollout was based on a recommendation by the National Commission on Terrorist Attacks Upon the United States.

The "9-11 Commission" made the suggestion as part of its final report in 2004, leading to congressional debate and delayed deadlines for the following 13 years, as reported then by The Atlantic, which also said “biometric systems of the past collected fingerprints. Today’s systems can be built to recognize individual faces, even voiceprints.”

Orlando International Airport was the first U.S. airport to commit to processing all arriving and departing international travelers with facial recognition technology in June 2018, according to the CBP.

Before that, a biometric screening pilot program started at one checkpoint at JFK International Airport in 2017.