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Federal law enforcement has warned America’s airports and airlines that they remain top targets for terrorists because of their symbolic value, "inherent accessibility, and the presence of large crowds in unsecure areas," according to a new government memo obtained by NBC News.

The "law enforcement sensitive" memo, sent Tuesday to the nation's passenger airlines, law enforcement agencies and many of the nation's nearly 20,000 airports, reminds the airline industry that "terrorists outside the U.S. remain intent on attacking airports."

Written by the FBI, Department of Homeland Security, and the National Counterterrorism Center, the nine-page document comes on the heels of a new ban on passengers carrying electronic devices aboard flights from some foreign airports because of terror fears, but does not address that threat.

The memo, however, does address recent mass casualty terror attacks on foreign airports and airliners, including deadly airport attacks in Turkey and Belgium and the downing of a passenger jet over Egypt in 2015 via a bomb allegedly planted by ISIS.

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The threats listed in the memo include:

- "Artfully concealed weapons and explosives" loaded onto both passenger and cargo airlines.

- The "insider threat" posed by airport employees giving outsiders access to secure areas and planes. "In addition, cybersecurity should be part of any insider threat program, which may include appropriate vetting of personnel with access to critical or secure systems and users with account-management privileges."

- Drones. The memo reminds first responders to report suspicious drone activity.

The memo suggests additional security measures for the industry, including:

- Providing security "training to service providers, such as taxi/cab drivers, cleaners, cargo carriers, and parking attendants, to serve as force multipliers in identifying and reporting suspicious activity."

- The use of additional technology to monitor sensitive areas. For instance, "consider using license plate readers at key locations, such as departures, arrivals, and parking garages."

- Using a guard tower or towers along the perimeter of an airport so there is an elevated line of sight of the airport’s perimeter.

The document also examines tools airport officials can use to communicate with the public about terror threats, including a cellphone alerting system that can send messages to phones near and in the airport.