Federal officials said Thursday they will review how airlines protect personal information about their passengers and whether they are making money by sharing that information with other parties.

The U.S. Department of Transportation said its review will focus on the 10 biggest U.S. airlines and cover their collection, handling and use of information about customers.

"Airline passengers should have confidence that their personal information is not being shared improperly with third parties or mishandled by employees," Transportation Secretary Pete Buttigieg said.

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A spokeswoman for the trade group Airlines for America said, "U.S. airlines take customers’ personal information security very seriously, which is why they have robust policies, programs and cybersecurity infrastructure to protect consumers’ privacy."

In announcing the review, the Transportation Department did not make allegations against any of the carriers or cite any events that might have prompted the move. A spokesman said it is being done "proactively" to help the department determine how to protect passengers' information.

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The department said it sent letters to each of the airlines — Delta, United, American, Southwest, Alaska, JetBlue, Spirit, Frontier, Hawaiian and Allegiant — about their procedures for collecting and using passenger information, including "monetization of passenger data, targeted advertising, and prevention of data breaches."

The agency also asked airlines if they have received complaints about employees or contractors mishandling personal information.

Delta, United, American, Southwest and Alaska referred questioners to the Airlines for America statement. Allegiant, which is not part of the trade group, said protecting customer data is a priority, and it welcomes the government review.