Here’s a look at what is already in place.

Preclearance

The preclearance program is run by the Customs and Border Protection agency. Customs officers are stationed at 16 airports around the world and screen passengers before they board planes to the United States. Customs officers based at foreign airports collect fingerprints and photos and check travel documents before allowing passengers to travel to the United States. The officers also conduct inspection interviews with passengers and monitor their behavior before they are allowed to fly.

According to the Government Accountability Office, data from the agency show that it identified and stopped over 22,000 high-risk air travelers in the fiscal year 2015 through these predeparture programs. The customs officers at these locations determined that 10,648 of the approximately 16 million air travelers seeking admission to the United States through such locations were inadmissible. Many of those who were denied entry onto flights bound for the United States were stopped for national security reasons.

The Immigration Advisory Program

Created in 2004, the Immigration Advisory Program is intended to prevent terrorists and high-risk passengers from boarding commercial aircraft to the United States. Unlike in the preclearance programs, customs officers in this program are unarmed, plainclothes officers who discreetly help airline and security employees with a review of passenger reservation and ticketing data on flights bound for the United States.

These officers also assist foreign countries with document examination to check for fraud and provide training for airlines and foreign security personnel. The officers make nonbinding “no board” recommendations to air carriers and the host governments to prevent these passengers from boarding flights to the United States. Airlines typically adhere to the recommendations, even though they are not required to. These customs officers serve strictly as advisers in the foreign countries. They hold no law enforcement authority.